Display device and method for driving the same

ABSTRACT

A semiconductor device having a configuration hardly generating variations in the current value due to a deteriorated EL element is to be provided. A capacitance element is disposed between the gate and the source of a driving TFT, video signals are inputted to the gate electrode, and then it is in the floating state. Suppose an EL element is deteriorated and the anode potential rises, that is, the source potential of the driving TFT rises, the potential of the gate electrode of the driving TFT, being in the floating state by coupling of the capacitance element, is to rise by the same amount. Accordingly, even when the anode potential rises due to the deteriorated EL element, the rise is added to the gate electrode potential as it is, and the gate-source voltage of the driving TFT is allowed to be constant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/814,521, filed Nov. 16, 2017, now allowed, which is a continuation ofU.S. application Ser. No. 13/855,753, filed Apr. 3, 2013, now U.S. Pat.No. 9,825,068, which is a continuation of U.S. application Ser. No.13/555,247, filed Jul. 23, 2012, now U.S. Pat. No. 8,508,443, which is acontinuation of U.S. application Ser. No. 10/291,736, filed Nov. 12,2002, now U.S. Pat. No. 8,242,986, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2001-348032 on Nov.13, 2001, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the configuration of a semiconductordevice having a transistor. The invention also relates to theconfiguration of an active matrix display device including asemiconductor device having a thin film transistor (hereafter, it isdenoted by TFT) fabricated on an insulator such as glass and plastics.In addition, the invention relates to an electronic device using suchthe display device.

In recent years, the development of display devices using light emittingto elements including electroluminescent (EL) elements has beenconducted actively. The light emitting element has high visibilitybecause it emits light for itself. It does not need a back light that isneeded in liquid crystal display devices (LCD), and thus it is suitablefor forming to have a low profile and has nearly no limits to the fieldof view.

Here, the EL element is an element having a light emitting layer thatcan obtain luminescence generated by applying an electric filed. Thelight emitting layer has light emission (fluorescence) in returning fromthe singlet excited state to the ground state, and light emission(phosphorescence) in returning from the triplet excited state to theground state. In the invention, the light emitting device may have anylight emission forms above.

The EL element is configured in which the light emitting layer issandwiched between a pair of electrodes (an anode and a cathode),forming a laminated structure in general. Typically, the laminatedstructure of the anode/hole transport layer/emissive layer/electrontransport layer/cathode is named, which was proposed by Tang et al.,Eastman Kodak Company. This structure has significantly high luminousefficiency, which is adapted to many EL elements now underinvestigation.

Furthermore, there are the other structures laminated between an anodeand a cathode in the order of the hole injection layer/hole transportlayer/light emitting layer/electron transport layer, or hole injectionlayer/hole transport layer/light emitting layer/electron transportlayer/electron injection layer. As the EL element structure used for thelight emitting device in the invention, any structure described abovemay be adapted. Moreover, fluorescent dyes may be doped into the lightemitting layer.

In the specification, the entire layers disposed between the anode andthe cathode are collectively called the EL layer in the EL element.Accordingly, the hole injection layer, the hole transport layer, thelight emitting layer, the electron transport layer, and the electroninjection layer are all included in the EL element. The light emittingelement formed of the anode, the EL layer, and the cathode is called ELelement.

FIGS. 2A and 2B depict the configuration of a pixel in a general lightemitting device. In addition, as the typical light emitting device, anEL display device is exemplified. The pixel shown in FIGS. 2A and 2B hasa source signal line 201, a gate signal line 202, a switching TFT 203, adriving TFT 204, a capacitance element 205, a current supply line 206,an EL element 207, and a power source line 208. The P-channel type isused for the driving TFT 204 in FIG. 2A, and the N-channel type is usedfor the driving TFT 204 in FIG. 2B. The switching TFT 203 is a TFT thatfunctions as a switch in inputting video signals to the pixel, and thusthe polarity is not defined.

The connection of each part will be described. Here, the TFT has threeterminals, the gate, the source and the drain, but the source and thedrain cannot differ from each other distinctly because of the structureof the TFT. Therefore, in describing the connection between theelements, one of the source and the drain is denoted by a firstelectrode, and the other is a second electrode. When the description isneeded for potential of each terminal (the gate-source voltage of acertain TFT) about turning on and off the TFT, the source and the drainare denoted.

Furthermore, in the specification, the TFT being on is the state thatthe gate-source voltage of the TFT exceeds the threshold and current iscarried between the source and the drain. The TFT being off is the statethat the gate-source voltage of the TFT drops below the threshold andcurrent is not carried between the source and the drain.

The gate electrode of the switching TFT 203 is connected to the gatesignal line 202, the first electrode of the switching TFT 203 isconnected to the source signal line 201, and the second electrode of theswitching TFT 203 is connected to the gate electrode of the TFT drivingTFT 204. The first electrode of the driving TFT 204 is connected to thecurrent supply line 206, and the second electrode of the driving TFT 204is connected to the anode of the EL element 207. The cathode of the ELelement 207 is connected to the power source line 208. The currentsupply line 206 and the power source line 208 have the potentialdifference each other. Moreover, to hold the gate-source voltage of thedriving TFT 204, a certain fixed potential, the capacitance element 205may be disposed between the gate electrode of the driving TFT 204 andthe current supply line 206, for example.

When a pulse is inputted to the gate signal line 202 to turn on theswitching TFT 203, video signals having been outputted to the sourcesignal line 201 are inputted to the gate electrode of the driving TFT204. The gate-source voltage of the driving TFT 204 is determined inaccordance with the potential of the inputted video signals, and thecurrent carried between the source and drain of the driving TFT 204(hereafter, it is denoted by drain current) is determined. This currentis supplied to the EL element 207 to emit light.

SUMMARY OF THE INVENTION

The display device in which TFTS are formed on a substrate and a pixelpart and peripheral circuits are built in one piece is applied to mobiledevices in significant growth, taking advantage of its small size andlightweight. At the same time, TFTs are formed through many processessuch as film deposition, device fabrication by repeating etching, andinjection of impurity elements for giving conductivity tosemiconductors, thus having a challenge of cost reduction by curtailingthe processes.

Then, when the pixel part and the peripheral circuits are configured ofunipolar TFTs, a part of the process of injecting impurity elements canbe omitted. As an example of a pixel formed by using unipolar TFTs, thepixel shown in FIG. 8 is proposed in Amorphous Silicon Thin-FilmTransistors Based Active-Matrix Organic Light-Emitting Displays, ASIADISPLAY, page 315, (2001).

The pixel shown in FIG. 8 has a source signal line 801, a gate signalline 802, a switching TFT 803, a driving TFT 804, an active resistanceTFT 805, a capacitance element 806, a current supply line 807, ELelement 808, and a power source line 809, using the N-channel TFT forthe TFTs 803 to 805.

The gate electrode of the switching TFT 803 is connected to the gatesignal line 802, the first electrode of the switching TFT 803 isconnected to the source signal line 801, and the second electrode of theswitching TFT 803 is connected to the gate electrode of the driving TFT804. The first electrode of the driving TFT 804 is connected to theanode of the EL element 808, and the second electrode of the driving TFT804 is connected to the first electrode of the active resistance TFT805. The gate electrode and the second electrode of the activeresistance TFT 805 are connected each other, which are connected to thecurrent supply line 807. The cathode of the EL element 808 is connectedto the power source line 809, having the potential difference with thecurrent supply line 807 each other. The capacitance element 806 isdisposed between the gate electrode of the driving TFT 804 and thecurrent supply line 807, holding the potential of signals applied to thegate electrode of the driving TFT 804.

As shown in FIGS. 2A and 8, the operation of using the N-channel TFT forthe driving TFT will be considered. FIG. 2C depicts only the configuredportion of the current supply line 206 to the driving TFT 204 to the ELelement 207 to the power source line 208 in the pixel shown in FIGS. 2Aand 2B. The driving TFT 204 is formed to be the N-channel type, and thusone side connected to the anode of the EL element 207 is the source, andthe other side connected to the current supply line is the drain.

Now, suppose the potential of the current supply line 206 is V_(DD), theanode potential of the EL element 207 is V_(A), the cathode potentialthereof is V_(C), and the potential of the gate electrode of the drivingTFT 204 is V_(Sig). The gate-source voltage V_(GS) of the driving TFT204 is V_(GS)=(V_(Sig)−V_(A)), and the anode-cathode voltage V_(EL) ofthe EL element 207 is V_(EL)−(V_(A)−V_(C)).

FIG. 2D depicts the voltage-current characteristics of the driving TFT204 and the EL element 207. The intersection of the voltage-currentcurve of the driving TFT 204 and the voltage-current curve of the ELelement 207 is the operating point, determining the current valuecarried through the EL element 207 and the anode potential V_(A) of theEL element. Now, when the voltage-current curve of the EL element 207 isexpressed by 211 and the voltage-current curve of the TFT 204 isexpressed by 213, the operating point falls into 215, whereby thecurrent value and V_(A)=_(VA1) are determined. In addition, thegate-source voltage V_(GS) of the driving TFT 204 at this time isexpressed by V_(GS)=(V_(Sig)−V_(A1)).

Here, the case of the EL element 207 having been deteriorated will beconsidered. When the EL element 207 is deteriorated, the voltage tostart lighting rises, the curve is shifted to the right and expressed by212. Here, suppose the driving TFT 204 is operated in the saturationregion and the deteriorated EL element 207 does not cause thegate-source voltage to be varied, the operating point shifts to 216.More specifically, it turns to be V_(A)=V_(A2). In this case, eventhough the source-drain voltage of the driving TFT 204 is varied, thecurrent value is not varied greatly, and thus the luminance is notvaried so much. However, at present, the N-channel TFT is used for thedriving TFT 204 and the one side connected to the anode of the ELelement 207 is the source. Thus, the gate-source voltage V_(GS) of thedriving TFT 204 becomes as small as V_(GS)=(V_(Sig)−V_(A2)). Therefore,the voltage-current curve of the driving TFT 204 at this time isexpressed by 214. Accordingly, the operating point falls into 217. Morespecifically, the deteriorated EL element 207 caused the sourcepotential of the driving TFT 204 to rise and the gate-source voltage tobe small, and thus the current value is changed greatly, leading to thedecrease in the luminance.

In the invention, the object is to provide a semiconductor device, inwhich the N-channel TFT is used for the driving TFT for supplyingcurrent to the EL element, capable of solving the problems caused by thedeteriorated EL element as described above.

The main point of the above-described object is the deteriorated ELelement caused the anode potential of EL element, namely, the sourcepotential of the driving TFT to rise and therefore the gate-sourcevoltage of the driving TFT to be small.

In order to make the current value not to be varied when the EL elementis deteriorated, it is necessary to make the gate-source voltage of thedriving TFT not to be varied when the deteriorated EL element causes theanode potential of EL element to rise.

In the invention, a structure adopting a bootstrap operation is appliedto the pixel. A capacitance element is provided between the gate and thesource of the driving TFT, and the source potential is set to a certainvalue during a period that the image signals are inputted to the gateelectrodes. After the image signals are inputted, the gate electrodesare in a floating state. At this time, if the source-gate voltage of thedriving TFT is in excess of the threshold value, the driving TFT isturned to ON. However, if the set source potential of the driving TFT isreleased, the current flows to the EL element, as a result, the anodepotential, namely, the source potential of the driving TFT rises.Accordingly, the potential of the gate electrodes in a state offloating, by coupling of the capacitance element disposed between thegate and the source of the driving TFT, is to rise by the same amount.As a result, when the anode potential rises variously due to thedeterioration of EL element, the rise can be added over to the potentialof the gate electrodes as it is, and the gate-source voltage of thedriving TFT is allowed to be constant thereby.

The ability of the capacitance element (storage capacitor) is explained.The gate potential of the driving TFT to which the image signals havebeen inputted is changed by a leak current of transistors or the like,and the source-gate voltage of the driving TFT is changed. As a result,the drain current of the driving TFT is changed, and the luminance isdecreased. That is, the capacitance element needs the ability to holdthe charge to set the gate potential of the driving TFT at a constantvalue or an almost constant value for a predetermined display period.

The configuration of the present invention is described as below.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has first and second switching elements having twostates, a conducting state and a non-conducting state, a transistor, acapacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source, and

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has first, second and third switching elements havingtwo states, a conducting state and a non-conducting state, a transistor,a capacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source,

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor, and

a first electrode of the third switching element is electricallyconnected to the gate electrode of the transistor, and a secondelectrode of the third switching element is so electrically connected toany one of the first electrode of the transistor, the second powersource, and the third power source.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has first, second and third switching elements havingtwo states, a conducting state and a non-conducting state, a transistor,a capacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source,

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor, and

a first electrode of the third switching element is electricallyconnected to the first electrode of the light emitting element, and asecond electrode of the third switching element is electricallyconnected to the second power source.

A semiconductor device of the invention comprising a pixel having alight emitting element,

wherein the pixel has first, second and third switching having twostates, a conducting state and a non-conducting state, a transistor, acapacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source through the thirdswitching element,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source, and

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor.

In the semiconductor device of the invention, when a conductivity typeof the transistor is an N-channel type, potential V₁ of the first powersource, potential V₂ of the second power source and potential V₃ of thethird power source can be V₁>V₂ and V₁>V₃.

In the semiconductor device of the invention, the potential V₂ of thesecond power source and the potential V₃ of the third power source alsocan be V₂<V₃.

In the semiconductor device of the invention, when a conductivity typeof the transistor is a P-channel type, potential V₁ of the first powersource, potential V₂ of the second power source, and potential V₃ of thethird power source can be V₁<V₂ and V₁<V₃.

In the semiconductor device of the invention, the potential V₂ of thesecond power source and the potential V₃ of the third power source alsocan be V₂>V₃.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has a source signal line, first and second gate signallines, a current supply line, first, second, and third transistors, acapacitance element, and the light emitting element,

a gate electrode of the first transistor is electrically connected tothe first gate signal line, a first electrode of the first transistor iselectrically connected to a first electrode of the second transistor anda first electrode of the light emitting element, and a second electrodeof the first transistor is electrically connected to a first powersource having a potential difference with the current supply line eachother, or the first or second gate signal line in any one of pixels notincluding the pixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe second gate signal line, and a second electrode of the thirdtransistor is electrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line each other, and

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has a source signal line, a gate signal line, acurrent supply line, first, second, and third transistors, a capacitanceelement, and the light emitting element,

a gate electrode of the first transistor is electrically connected tothe gate signal line, a first electrode of the first transistor iselectrically connected to a first electrode of the second transistor anda first electrode of the light emitting element, and a second electrodeof the first transistor is electrically connected to a first powersource having a potential difference with the current supply line eachother or the gate signal line in any one of pixels not including thepixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe gate signal line, and a second electrode of the third transistor iselectrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line each other, and

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has a source signal line, first, second and third gatesignal lines, a current supply line, first, second, third and fourthtransistors, a capacitance element, and the light emitting element,

a gate electrode of the first transistor is electrically connected tothe first gate signal line, a first electrode of the first transistor iselectrically connected to a first electrode of the second transistor anda first electrode of the light emitting element, and a second electrodeof the first transistor is electrically connected to any one of a firstpower source having a potential difference with the current supply lineeach other, the first, second and third gate signal lines in any one ofpixels not including the pixel, and the second and third gate signallines in the pixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe second gate signal line, and a second electrode of the thirdtransistor is electrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line each other,

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor, and

a gate electrode of the fourth transistor is electrically connected tothe third gate signal line, a first electrode of the fourth transistoris electrically connected to the gate electrode of the secondtransistor, and a second electrode of the fourth transistor iselectrically connected to any one of the first electrode of the secondtransistor, the first power source, and the second power source.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has a source signal line, first and second gate signallines, a current supply line, first, second, third and fourthtransistors, a capacitance element, and the light emitting element

a gate electrode of the first transistor is electrically connected tothe first gate signal line, a first electrode of the first transistor iselectrically connected to a first electrode of the second transistor anda first electrode of the light emitting element, and a second electrodeof the first transistor is electrically connected to any one of a firstpower source having a potential difference with the current supply lineeach other, the first and second gate signal lines in any one of pixelsnot including the pixel, and the second gate signal line in the pixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe first gate signal line, and a second electrode of the thirdtransistor is electrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line each other,

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor, and

a gate electrode of the fourth transistor is electrically connected tothe second gate signal line, a first electrode of the fourth transistoris electrically connected to the gate electrode of the secondtransistor, and a second electrode of the fourth transistor iselectrically connected to any one of the first electrode of the secondtransistor, the first power source, and the second power source.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has a source signal line, first, second and third gatesignal lines, a current supply line, first, second, third and fourthtransistors, a capacitance element, and the light emitting element,

a gate electrode of the first transistor is electrically connected tothe first gate signal line, a first electrode of the first transistor iselectrically connected to a first electrode of the second transistor anda first electrode of the light emitting element, and a second electrodeof the first transistor is electrically connected to any one of a firstpower source having a potential difference with the current supply lineeach other, the first, second and third gate signal lines in any one ofpixels not including the pixel, and the second and third gate signallines in the pixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe second gate signal line, and a second electrode of the thirdtransistor is electrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line each other,

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor, and

a gate electrode of the fourth transistor is electrically connected tothe third gate signal line, a first electrode of the fourth transistoris electrically connected to the first electrode of the light emittingelement, and a second electrode of the fourth transistor is electricallyconnected to the first power source.

A semiconductor device is characterized by comprising a pixel having alight emitting element,

wherein the pixel has a source signal line, first and second gate signallines, a current supply line, first, second, third and fourthtransistors, a capacitance element, and the light emitting element,

a gate electrode of the first transistor is electrically connected tothe first gate signal line, a first electrode of the first transistor iselectrically connected to a first electrode of the second transistor anda first electrode of the light emitting element, and a second electrodeof the first transistor is electrically connected to any one of a firstpower source having a potential difference with the current supply lineeach other, the first, second and third gate signal lines in any one ofpixels not including the pixel, and the second and third gate signallines in the pixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe first gate signal line, and a second electrode of the thirdtransistor is electrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line each other,

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor, and

a gate electrode of the fourth transistor is electrically connected tothe second gate signal line, a first electrode of the fourth transistoris electrically connected to the first electrode of the light emittingelement, and a second electrode of the fourth transistor is electricallyconnected to the first power source.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has a source signal line, first, second and third gatesignal lines, a current supply line, first, second, third and fourthtransistors, a capacitance element, and the light emitting element,

a gate electrode of the first transistor is electrically connected tothe first gate signal line, a first electrode of the first transistor iselectrically connected to a first electrode of the second transistor anda first electrode of the light emitting element, and a second electrodeof the first transistor is electrically connected to any one of a firstpower source having a potential difference with the current supply lineeach other, the first, second and third gate signal lines in any one ofpixels not including the pixel, and the second and third gate signallines in the pixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe second gate signal line, and a second electrode of the thirdtransistor is electrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line,

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor, the capacitance element holdsvoltage between the gate electrode and the first electrode of the secondtransistor, and

the fourth transistor is disposed between the second electrode of thesecond transistor and the current supply line, or between the firstelectrode of the second transistor and the first electrode of the lightemitting element, and a gate electrode of the fourth transistor iselectrically connected to the third gate signal line.

A semiconductor device of the invention is characterized by comprising apixel having a light emitting element,

wherein the pixel has a source signal line, first and second gate signallines, a current supply line, first, second, third and fourthtransistors, a capacitance element, and the light emitting element,

a gate electrode of the first transistor is electrically connected tothe first gate signal line, a first electrode of the first transistor iselectrically connected to a first so electrode of the second transistorand a first electrode of the light emitting element, and a secondelectrode of the first transistor is electrically connected to any oneof a first power source having a potential difference with the currentsupply line each other, the first and second gate signal lines in anyone of pixels not including the pixel, and the second gate signal linein the pixel,

a gate electrode of the second transistor is electrically connected to afirst electrode of the third transistor, and a second electrode of thesecond transistor is electrically connected to the current supply line,

a gate electrode of the third transistor is electrically connected tothe first gate signal line, and a second electrode of the thirdtransistor is electrically connected to the source signal line,

a second electrode of the light emitting element is electricallyconnected to a second power source having a potential difference withthe current supply line each other,

the capacitance element is disposed between the gate electrode and thefirst electrode of the second transistor, the capacitance element holdsvoltage between the gate electrode and the first electrode of the secondtransistor, and

the fourth transistor is disposed between the second electrode of thesecond transistor and the current supply line, or between the firstelectrode of the second transistor and the first electrode of the lightemitting element, and a gate electrode of the fourth transistor iselectrically connected to the third gate signal line.

In a semiconductor device of the invention, the first and thirdtransistors can be the same conductive type.

In a semiconductor device of the invention, the transistors included inthe pixel can be the same conductive type.

In a semiconductor device of the invention, when a conductive type ofthe second transistor is an N-channel type, potential V₁ of the currentsupply line, potential V₂ of the first power source, and potential V₃ ofthe second power source are V₁>V₂ and V₁>V₃.

In a semiconductor device of the invention, when the conductive type ofthe second transistor is the N-channel type, the potential V₂ of thefirst power source, and the potential V₃ of the second power source areV₂>V₃.

In a semiconductor device of the invention, when the conductive type ofthe second transistor is a P-channel type, potential V₁ of the currentsupply line, potential V₂ of the first power source, and potential V₃ ofthe second power source are V₁<V₂ and V₁<V₃.

In a semiconductor device of the invention, when the conductive type ofthe second transistor is the P-channel type, the potential V₂ of thefirst power source, and the potential V₃ of the second power source areV₂<V₃.

A method for driving a semiconductor device of the invention ischaracterized by that a pixel having a light emitting element isdisposed,

wherein the pixel has first and second switching elements having twostates, a conducting state and a non-conducting state, a transistor, acapacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source,

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor,

the method for driving the display device comprising:

a first step of conducting the first and second switching elements toinput the video signal to the gate electrode of the transistor, andfixing potential of the first electrode of the transistor;

a second step of not conducting the first and second switching elementsto allow the gate electrode of the transistor to be in a floating state;and

a third step of supplying current corresponding to potential applied tothe gate electrode of the transistor to the light emitting element toemit light,

wherein in the third step, the capacitance element holds gate-sourcevoltage of the transistor to allow a potential variation of the firstelectrode of the transistor to be equal to a potential variation of thegate electrode of the transistor.

A method for driving a semiconductor device of the invention ischaracterized by that a pixel having a light emitting element isdisposed,

wherein the pixel has first, second and third switching elements havingtwo states, a conducting state and a non-conducting state, a transistor,a capacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source,

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor, and

a first electrode of the third switching element is electricallyconnected to the gate electrode of the transistor, and a secondelectrode of the third switching element is electrically connected toany one of the first electrode of the transistor, the second powersource, and the third power source,

the method for driving the display device comprising:

a first step of conducting the first and second switching elements toinput the video signal to the gate electrode of the transistor, andfixing potential of the first electrode of the transistor,

a second step of not conducting the first and second switching elementsto allow the gate electrode of the transistor to be in a floating state;

a third step of supplying current corresponding to potential applied tothe gate electrode of the transistor to the light emitting element toemit light; and

a fourth step of conducting the third switching element to allowgate-source voltage of the transistor to be equal to or below anabsolute value of a threshold voltage, and stopping current supply tothe light emitting element,

wherein in the third step, the capacitance element holds the gate-sourcevoltage of the transistor to allow a potential variation of the firstelectrode of the transistor to be equal to a potential variation of thegate electrode of the transistor.

A method for driving a semiconductor device of the invention ischaracterized by that a pixel having a light emitting element isdisposed,

wherein the pixel has first, second and third switching elements havingtwo states, a conducting state and a non-conducting state, a transistor,a capacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source,

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor, and

a first electrode of the third switching element is electricallyconnected to the first electrode of the light emitting element, and asecond electrode of the third switching element is electricallyconnected to the second power source,

the method for driving the display device comprising:

a first step of conducting the first and second switching elements toinput the video signal to the gate electrode of the transistor, andfixing potential of the first electrode of the transistor,

a second step of not conducting the first and second switching elementsto allow the gate electrode of the transistor to be in a floating state;

a third step of supplying current corresponding to potential applied tothe gate electrode of the transistor to the light emitting element toemit light; and

a fourth step of conducting the third switching element to allowgate-source voltage of the transistor to be equal to or below anabsolute value of a threshold voltage, and stopping current supply tothe light emitting element,

wherein in the third step, the capacitance element holds the gate-sourcevoltage of the transistor to allow a potential variation of the firstelectrode of the transistor to be equal to a potential variation of thegate electrode of the transistor.

A method for driving a semiconductor device of the invention ischaracterized by that a pixel having a light emitting element isdisposed,

wherein the pixel has first, second, and third switching elements havingtwo states, a conducting state and a non-conducting state, a transistor,a capacitance element, and the light emitting element,

a video signal is inputted to a first electrode of the first switchingelement, and a second electrode of the first switching element iselectrically connected to a gate electrode of the transistor,

a first electrode of the transistor is electrically connected to a firstelectrode of the second switching element and a first electrode of thelight emitting element, and a second electrode of the transistor iselectrically connected to a first power source through the thirdswitching element,

a second electrode of the second switching element is electricallyconnected to a second power source,

a second electrode of the light emitting element is electricallyconnected to a third power source, and

the capacitance element is disposed between the gate electrode and thefirst electrode of the transistor,

the method for driving the display device comprising:

a first step of conducting the first and second switching elements toinput the video signal to the gate electrode of the transistor, andfixing potential of the first electrode of the transistor;

a second step of not conducting the first and second switching elementsto allow the gate electrode of the transistor to be in a floating state;

a third step of conducting the third switching element to supply currentcorresponding to potential applied to the gate electrode of thetransistor to the light emitting element to emit light; and

a fourth step of not conducting the third switching element and stoppingcurrent supply to the light emitting element,

wherein in the third step, the capacitance element holds gate-sourcevoltage of the transistor to allow a potential variation of the firstelectrode of the transistor to be equal to a potential variation of thegate electrode of the transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams for illustrating an embodiment of theinvention and the operation;

FIGS. 2A to 2D are diagrams for illustrating the operation in the caseof forming the TFTs to be unipolar by the traditional configuration;

FIGS. 3A to 3C are diagrams for illustrating the operation of thecircuit according to the configuration shown in FIG. 1A;

FIGS. 4A to 4C are diagrams for illustrating an embodiment of theinvention and the operation;

FIGS. 5A to 5C are diagrams for illustrating an embodiment of theinvention and the operation;

FIGS. 6A to 6E are diagrams for illustrating an embodiment of theinvention and the operation;

FIGS. 7A to 7H are diagrams comparing the invention with the traditionalexample on the change in the potential around the gate electrode and thesource region of the driving TFT,

FIG. 8 is a diagram introducing one example of the pixel configured ofthe unipolar TFTs;

FIG. 9 is a diagram depicting an embodiment of the invention;

FIGS. 10A and 10B are diagrams for illustrating the time gray scalesystem;

FIGS. 11A to 11C are diagrams for illustrating the time gray scalesystem;

FIGS. 12A to 12D are diagrams for illustrating an embodiment of theinvention and the operation;

FIGS. 13A to 13D are diagrams for illustrating the fabrication processesof a semiconductor device;

FIGS. 14A to 14C are diagrams for illustrating the fabrication processesof the semiconductor device;

FIGS. 15A to 15C are a top view and cross sections of the semiconductordevice;

FIGS. 16A to 16C are diagrams depicting the configuration of asemiconductor device for display with analogue video signals;

FIGS. 17A and 17B are diagrams depicting an example of a source signalline drive circuit and a gate signal line drive circuit in the deviceshown in FIGS. 16A to 16C;

FIGS. 18A and 18B are diagrams depicting the configuration of asemiconductor device for display with digital video signals;

FIGS. 19A and 19B are diagrams depicting an example of a source signalline drive circuit in the device shown in FIGS. 18A and 18B;

FIGS. 20A to 20H are diagrams depicting examples of electronic devicesapplicable to the invention;

FIGS. 21A to 21C are diagrams for illustrating an embodiment of theinvention and the operation; and

FIG. 22 is a diagram depicting a top view of the pixel configuration ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment 1

FIG. 1A depicts an embodiment of the invention. The pixel of theinvention has a source signal line 101, a gate signal line 102, first,second and third TFTs 103 to 105, a capacitance element 106, a currentsupply line 107, an EL element 108, and power source lines 109 and 110.The gate electrode of the TFT 103 is connected to the gate signal line102, the first electrode of the TFT 103 is connected to the sourcesignal line 101, and the second electrode of the TFT 103 is connected tothe gate electrode of the TFT 104. The first electrode of the TFT 104 isconnected to the current supply line 107, and the second electrode ofthe TFT 104 is connected to the first electrode of the TFT 105 and thefirst electrode of the EL element. The gate electrode of the TFT 105 isconnected to the gate signal line 102, and the second electrode of theTFT 105 is connected to the power source line 110. The second electrodeof the EL element 108 is connected to the power source line 109. Thecapacitance element 106 is disposed between the gate electrode and thesecond electrode of the TFT 104, holding the gate-source voltage.

Now, all the TFTs 103 to 105 are the N-channel TFT, and they are to beturned on when the gate-source voltage exceeds the threshold. Inaddition, in the EL element 108, the first electrode is the anode, andthe second electrode is cathode. The anode potential is set V_(A), andthe cathode potential, i.e. the potential of the power source line 109is set V_(C). Furthermore, the potential of the current supply line 107is set V_(DD), and the potential of the power source line 110 is setV_(SS). The potential of the video signal is set V_(Sig).

The operation of the circuit will be described with FIGS. 1A, 1B and 3Ato 3C. Here, the gate (G), the source (S), and the drain (D) of the TFT104 is defined as shown in FIG. 3A.

The gate signal line 102 is selected in a certain pixel to turn on theTFTs 103 and 105. As shown in FIG. 3A, video signals are inputted to thegate electrode of the TFT 104 from the source signal line 101, and thepotential is turned to be V_(Sig). In the meantime, the TFT 105 is on,thus being V_(A)=V_(SS). At this time, when V_(SS)≤V_(C) is set, currentis not carried through the EL element 108 in writing the video signals.However, V_(SS)>V_(C) is set, and thus it is acceptable to carry currentthrough the EL element 108. The essence here is that V_(A) is fixed to afixed potential. According to this operation, the voltage between bothelectrodes of the capacitance element 106 is turned to be(V_(Sig)−V_(SS)). Then, when the select period of the gate signal line102 is finished and the TFTs 103 and 105 are turned off, the migrationpath of charges stored in the capacitance element 106 is gone, and thegate-source voltage (V_(Sig)−V_(SS)) of the TFT 104 is held (FIG. 3B).

Here, when (V_(Sig)−V_(SS)) exceeds the threshold of the TFT 104, theTFT 104 is turned on, current is started to carry through the EL elementfrom the current supply line 107, and light emission is started (FIG.3C), increasing the source potential of the TFT 104. At this period, thegate electrode of the TFT 104 is in the floating state, and thecapacitance element 106 holds the gate-source voltage of the TFT 104.Thus, the potential of the gate electrode is increased with the rise inthe source potential. At this period, the capacitance component existsbetween the gate electrode and the semiconductor layer (in the sourceregion or drain region) in the TFTS 104 and 105, but the capacitancevalue of the capacitance element 106 is set to be dominant sufficientlyover the capacitance component, whereby the rise in the source potentialof the TFT 104 is made nearly equal to the rise in the gate potential ofthe TFT 104.

Based on the operation, the operation according to the deteriorated ELelement or not will be considered with FIG. 1B. FIG. 1B schematicallydepicts that 151 is the potential of the gate signal line 102, 152 and153 are the potential V_(G) of the gate electrode of the TFT 104, 154and 155 are the anode potential V_(A) of the EL element 108, i.e. thesource potential of the TFT 104, and 156 is the gate-source voltageV_(GS) of the TFT 104.

Now, in the section expressed by i shown in FIG. 1B, the gate signalline 102 is selected to be at high level. Therefore, video signals arewritten in this section, and the gate potential V_(G) of the TFT 104rises. In the meantime, the TFT 105 is on, and thus the anode potentialV_(A) of the EL element 108, i.e. the source potential of the TFT 104becomes equal to V_(SS). Thus, the gate-source voltage V_(GS) of the TFT104 becomes larger. Furthermore, when it is V_(A)=V_(SS)<V_(C) in thesection, the EL element 108 does not emit light regardless of the valueof the video signal V_(Sig).

In the timing expressed by ii, the selection of the gate signal line 102is finished to be at low level, and the TFTs 103 and 105 are turned off.V_(GS)=(V_(Sig)−V_(A)) at this time is held in the capacitance element106.

Subsequently, go into the section expressed by iii, and the lightemission is started. At this time, when the gate-source voltage V_(GS)of the TFT 104 exceeds the threshold, the TFT 104 is turned on to carrythe drain current, and the EL element 108 emits light. At the same time,the source potential of the TFT 104 also rises. Here, as describedabove, the gate electrode of the TFT 104 is in the floating state, andthus the potential rises as similar to the rise in the source potentialof the TFT 104.

Here, the case where the EL element 108 has been deteriorated will beconsidered. When the EL element is deteriorated, anode-cathode voltagebecomes large in carrying a current of a certain value through the ELelement 108 as described above. Thus, V_(A) rises as expressed by 155.However, in the invention, V_(G) also rises by the rise of V_(A), andconsequently, it is revealed that V_(GS) is not changed.

On the other hand, as shown in FIGS. 7A to 7H, in the case of thetraditional configuration shown in FIG. 2B, when video signals are onceinputted and the potential is turned to be V_(Sig), the gate potentialV_(G) of the TFT 204 is not changed after that. Therefore, the ELelement 207 is deteriorated and V_(A) rises, the gate-source voltage ofthe TFT 204 becomes smaller than before deteriorated (FIGS. 7G and 7H).In such the case, even though the TFT 204 is operated in the saturationregion, the current value at the operating point is to be changed.Accordingly, when the EL element 207 is deteriorated and thevoltage-current characteristics are changed, the current carried throughthe EL element 207 becomes smaller to cause the luminance to bedecreased.

As described above, the current value is not changed even in thedeterioration of the EL element, whereby the invention can eliminate theinfluence of the deterioration of the EL element.

In addition, both the potential V_(SS) and V_(C) of the power sourcelines can be set arbitrarily. Therefore, V_(SS)<V_(C) is set, wherebythe reverse bias can be easily applied to the EL element.

Furthermore, TFTs 103 and 105 are fine to simply function as theswitching element, and thus the polarity is not defined. Morespecifically, even though all the TFTs configuring the pixel are set tobe unipolar, the normal operation is feasible. In FIG. 1A, the TFTs 103and 105 are set to have the same polarity and are controlled only by thegate signal line 102. However, it is acceptable that first and secondgate signal lines different from each other are used to control theseparate TFTs. In this case, the TFTs 103 and 105 may have the differentpolarity each other. However, in consideration of the numerical apertureof the pixel, the number of lines for wiring is desired to be a smallernumber as much as possible.

Embodiment 2

According to the configuration shown in FIG. 1A, for the lines routed tothe pixel part, five lines were needed: the source signal line, the gatesignal line, the current supply line (V_(DD)), the power source line(V_(C)), and the power source line (V_(SS)). In this embodiment, theconfiguration will be described in which lines for wiring are shared,whereby allowing the number of lines for wiring per pixel to be reducedand high numerical aperture to be obtained.

FIG. 9 depicts the configuration of the embodiment. The point differentfrom the embodiment 1 is only the point in that the second electrode ofa TFT 906 is connected to the power source line (V_(SS)) but it isconnected to the gate signal line in a pixel of the next row in theembodiment. Suppose the pixel expressed by a dotted frame 900 is in theith row, the second electrode of the TFT 906 is connected to the gatesignal line in the i+1st row.

As the pulse condition of selecting the gate signal line, it isacceptable that the gate-source voltage of the TFT 904 sufficientlyexceeds the threshold at high level. More specifically, it is acceptablethat the potential is sufficiently larger than the threshold to themaximum value of the video signal V_(Sig). In the meantime, thepotential is fine to surely turn off the TFT 904 at low level.Accordingly, the potential at low level is set equal to V_(SS) in thegate signal line.

When the ith gate signal line is selected to be at high level and theTFTs 904 and 906 are turned on, the i+1 st gate signal line is notselected yet. More specifically, it is at low level and the potential isV_(SS). Therefore, the anode potential V_(A) of the EL element becomesequal to V_(SS) through the TFT 906 as similar to the embodiment.Accordingly, when the lines for wiring are shared in accordance with theembodiment, the same effect as the embodiment 1 can be attained.

In addition, where to connect the second electrode of the TFT 906 is notlimited to the i+1st gate signal line, when it is a position where afixed potential V_(SS) can be applied while the ith gate signal line isselected to be at high level and the TFT 906 is on. For example, it maybe the i−1st gate signal line or other than this. When the signal linesin the adjacent rows are shared, the pulse of the signal lines aredesired not to be overlapped each other.

Furthermore, as described in the embodiment 1, the TFTs 904 and 906 arefine to simply function as the switching elements. Thus, the polarity isnot defined, which is not limited to being controlled by a single gatesignal line 902 as shown in FIG. 9.

Embodiment 3

It is called the analogue gray scale system that the gate-source voltageof a driving TFT is controlled, and the current value carried through anEL element is controlled by analogue quantity for display. In themeantime, the digital gray scale system is proposed in which an ELelement is driven only by two states, a hundred or zero percentluminance. In this system, only two levels of gray scale, black andwhite, can be displayed, but it has a merit of hardly being subject tovariations in the TFT characteristics. To intend to have the multiplegray scale by the digital gray scale system, a driving method ofcombining with the time gray scale system is used. The time gray scalesystem is the method of expressing the gray scale by the length of timethat the element emits light for a long time or short time.

When the digital gray scale system is combined with the time gray scalesystem, one frame period is split into a plurality of subframe periodsas shown in FIG. 10A. Each subframe period has the address (writing)period, the sustain (light emission) period, and the erase period, asshown in FIG. 10B. The gray scale is expressed in which the subframeperiods corresponding to the bit numbers for display are disposed, thelength of the sustain (light emission) period is set to2^((n-1)):2^((n-2)): . . . :2:1 in each subframe period, the EL elementis selected to emit light or not to emit light in each sustain (lightemission) period, and the difference in the length of the total timewhile the EL element is emitting light is utilized. The luminance ishigh when the time for emitting light is long, whereas the luminance islow when short. In addition, FIGS. 10A and 10B depict the example offour bit gray scale in which one frame period is split into foursubframe periods and 2⁴=16 levels of gray scale can be expressed by thecombination of the sustain (light emission) periods. Furthermore, thegray scale can be expressed without particularly setting the ratio ofthe length of the sustain periods to be the ratio of the powers of two.Moreover, a certain subframe period may be further split.

When the multiple gay scale is intended with the time gray scale system,the length of the sustain (light emission) period of lower bits becomesfurther shorter. Therefore, when the subsequent address period is tostart immediately after the sustain (light emission) period is finished,the period of overlapping with the address (writing) periods of thedifferent subframe periods is generated. In this case, video signalsinputted to a certain pixel are also inputted to the different pixel atthe same time, and thus the normal display cannot be performed. Theerase period is disposed for solving such the problem. As shown in FIG.10B, it is disposed after Ts3 and Ts4 so as not to overlap two differentaddress (writing) periods with each other. Accordingly, the erase periodis not disposed in SF1 and SF2 where the sustain (light emission) periodis long enough and the two different address (writing) periods will notoverlap with each other.

In this manner, to drive the EL element by the method of combining thedigital gray scale system with the time gray scale system, there mightbe the case of adding the operation that the light emission of the ELelement is forcedly stopped and the erase period is disposed.

FIG. 4A depicts an example of adding a second gate signal line 403 andan erasing TFT 407 to the pixel having the configuration shown in theembodiment 1 to respond to the driving method of combining the digitalgray scale system with the time gray scale system. The gate electrode ofthe erasing TFT 407 is connected to the second gate signal line 403, thefirst electrode of the erasing TFT 407 is connected to the gateelectrode of a TFT 405 and the first electrode of a capacitance element408, and the second electrode of the erasing TFT 407 is connected to thesecond electrode of the TFT 405 and the second electrode of thecapacitance element 408.

The operation that a first gate signal line 402 is selected to inputvideo signals is the same as that shown in the embodiment 1, thusomitting it here. In addition, during the input of the video signals,the second gate signal line is at low level and the erasing TFT 407 isoff. At this time, V_(Sig) takes either the potential to surely turn onthe TFT 405 or potential to turn off the TFT 405.

Here, the operation from the sustain (light emission) period to theerase period will be describe with FIGS. 4A to 4C and 11A to 11C. FIG.11A is the same as that shown in FIG. 10A. As shown in FIG. 11B, oneframe period has four subframe periods. In subframe periods SF3 and SF4having a short sustain (light emission) period, they have erase periodsTe3 and Te4, respectively. Here, the operation in SF3 will beexemplified for description.

After the video signals are finished to input, the current correspondingto the gate-source voltage V_(GS) of the TFT 405 is carried through anEL element 410 to emit light, as shown in FIG. 10B. Then, when timing isreached to finish the sustain (light emission) period, pulses areinputted to the second gate signal line 403 to be at high level, and theerasing TFT 407 is turned on to set the gate-source voltage V_(GS) ofthe TFT 405 to be zero, as shown in FIG. 4C. Accordingly, the TFT 405 isturned off, the current to the EL element 410 is broken, and the ELelement 410 forcedly stops light emission.

The operation is shown in FIG. 11C as a timing chart. The erase periodTe3 is the period that after the sustain (light emission) period Ts3, apulse is inputted to the second gate signal line 403, the EL element 410stops light emission, and then a pulse is again inputted to the firstgate signal line 402 to start inputting the next video signal.

In addition, in the configuration shown in FIG. 4A, the second electrodeof a TFT 406 is connected to a power source line 412, but the powersource line 412 can be substituted by the gate signal line in theadjacent row as shown in the embodiment 2. Furthermore, in theembodiment, the second gate signal line 403 is disposed for controllingthe erasing TFT 407, and thus the second electrode of the TFT 406 may beconnected to the second gate signal line 403.

Although the TFTs 404 and 406 are controlled by the gate signal line402, a new gate signal line may be added. In this case, the TFTs 404 and406 can be controlled by the gate signal line 402 and the newly addedgate signal line, respectively.

Embodiment 4

FIG. 5A depicts an example of disposing the erasing TFT at the positiondifferent from that shown in the embodiment 3. In this embodiment, anerasing TFT 507 is disposed between the gate electrode of a TFT 505 tothe first electrode of a capacitance element 508 and a power source line512.

The driving method is acceptable to be conducted by the method ofcombining the digital gray scale with the time gray scale systemregarding from the input of video signals to light emission as similarto the embodiment 3. Thus, the description is omitted here, and theoperation in the erase period will be described.

When timing is reached to finish the sustain (light emission) period, apulse is inputted to a second gate signal line 503 to be at high level,the erasing TFT 507 is turned on, and the potential of the gateelectrode of the TFT 505 is turned to be V_(SS), as shown in FIG. 5C.More specifically, in the erase period, the gate-source voltage V_(GS)of the TFT 505 is fine to be set below the threshold.

The source potential of the TFT 505 is in the potential at least equalto or greater than V_(SS). Therefore, the operation of the erasing TFT507 allows the gate-source voltage V_(GS) of the TFT 505 to be V_(GS)≤0,and the TFT 505 is turned off. Accordingly, the erase period is theperiod that the EL element 510 stops light emission, a pulse is againinputted to a first gate signal line 502, and the next video signal isagain started to input.

In addition, in the configuration shown in FIG. 5A, the second electrodeof a TFT 506 is connected to the power source line 512, but the powersource line 512 can be substituted by the gate signal line in theadjacent row as shown in the embodiment 2. Furthermore, in theembodiment, the second gate signal line 503 is disposed for controllingthe erasing TFT 507. Thus, the second electrode of the TFT 506 may beconnected to the second gate signal line 503.

Although the TFTs 504 and 506 are controlled by the gate signal line502, a new gate signal line may be added. In this case, the TFTs 504 and506 can be controlled by the gate signal line 402 and the newly addedgate signal line, respectively.

Embodiment 5

FIG. 6A depicts an example of disposing the erasing TFT at the positiondifferent from that shown in the embodiments 3 and 4. In thisembodiment, an erasing TFT 607 is disposed between the first electrodeof a TFT 605 and a current supply line.

The operation of the circuit will be described. A first gate signal line602 is selected to be at high level, a TFT 604 is turned on, and videosignals are inputted to a pixel from a source signal line 601. In themeantime, a TFT 606 is also turned on to allow the anode potential V_(A)of an EL element 610 to be equal to V_(SS). At this time, whenV_(SS)≤V_(C) is set, current is not carried though the EL element 610 inwriting the video signals, and thus the TFT 607 is fine to be on or off.

When the input of video signals is finished and the first gate signalline 602 is not selected, the gate electrode of the TFT 605 is in thefloating state and the migration path for stored charges is blocked in acapacitance element 608. Thus, the gate-source voltage V_(GS) is held inthe capacitance element 608.

Subsequently, a second gate signal line 603 is selected to be at highlevel and the TFT 607 is turned on, whereby current is carried as shownin FIG. 6D, the anode potential V_(A) of the EL element 610 rises togenerate the potential difference with the cathode potential V_(C), andthe current is carried to emit light. In addition, it is acceptable thatthe TFT 607 is turned on from the state of inputting the video signals.In this case, at the moment that the first gate signal line 602 isturned not to be selected, current is supplied to the EL element 610through the TFTs 607 and 605, and the anode potential V_(A) of the ELelement 610 rises to generate the potential difference with the cathodepotential V_(C), carrying the current to emit light.

When timing is reached to finish the sustain (light emission) period,the second gate signal line 603 is not selected to be at low level, theTFT 607 is turned off, and the current path from a current supply line609 to the EL element 610 is blocked. Accordingly, the current is notcarried through the EL element 610 to stop light emission. After that,the erase period is the period that a pulse is again inputted to thefirst gate signal line 602 and the next video signal is started toinput.

In addition, the TFT 607 is fine to be disposed between the firstelectrode of the TFT 605 and the anode of the EL element 610. Morespecifically, it is acceptable that the TFT 607 is disposed in thecurrent path between the current supply line 609 and the EL element 610and the current supply to the EL element 610 can be cut during the eraseperiod.

Although the TFTs 604 and 606 are controlled by the gate signal line602, a new gate signal line may be added. In this case, the TFTs 604 and606 can be controlled by the gate signal line 602 and the newly addedgate signal line, respectively.

Embodiment 6

In the embodiments 3 to 5, the example of adding the TFT to dispose theerase period has been described, but in this embodiment, an example ofperforming the same operation will be described without adding theerasing TFT.

FIG. 21A depicts the configuration. The configuration is nearly similarto that shown in the embodiment 1, but the difference is in that TFTs2104 and 2106 are controlled by separate gate signal lines 2102 and2103, respectively.

As shown in FIG. 21B, in the sustain (light emission) period, acapacitance element 2107 fixes the gate-source voltage of a TFT 2105 andthe current so accompanying this is carried through an EL element 2109to emit light.

Subsequently, go to the erase period, and a pulse is inputted to thesecond gate signal line 2103 to turn on the TFT 2106. At this time, thepotential of a power source line 2111 connected to the second electrodeof the TFT 2106 is set lower than the cathode potential of the ELelement 2109, i.e. the potential of a power source line 2110, wherebycurrent is not carried through the EL element 2109. Accordingly, thecurrent at this time is carried as shown in FIG. 21C.

In addition, the gate signal line in the adjacent row may be used forthe power source line 2111 as described in the other embodiments.

Embodiment 7

The N-channel TFT has been used for the TFT for supplying current to theEL element. However, the invention can be implemented by using theP-channel TFT for the driving TFT. FIG. 12A depicts the exemplaryconfiguration.

The circuit configuration is the same as that using the N-channel TFTshown in FIG. 1A. However, the differences are in that the configurationof an EL element 1208 is reverse, one side connected to the secondelectrode of the TFT 1204 is the cathode, and the other side connectedto a power source line 1209 is the anode, and that the potential of acurrent supply line 1207 is V_(SS), the potential of a power source line1209 is V_(A), and the potential of a power source line 1210 is V_(DD).Here, it is V_(SS)<V_(D)D and V_(A)<V_(DD).

The operation of the circuit will be described with FIGS. 12B to 12D. Inaddition, the polarity of the TFTs is the P-channel type, a low level isinputted to the gate electrode to turn on the TFTs, and a high level isinputted to turn off the TFTs.

In a certain row, a gate signal line 1202 is selected to be at lowlevel, and TFTs 1203 and 1205 are turned on. As shown in FIG. 12B, videosignals are inputted to the gate electrode of the TFT 1204 from a sourcesignal line 1201, and the potential is turned to be V_(Sig). In themeantime, the TFT 1205 is on, and thus the cathode potential V_(C) ofthe EL element 1208 is turned to be V_(C)=V_(DD). At this time, whenV_(A)≤V_(DD) is set, current is not carried through the EL element 1208in writing the video signals. According to this operation, the voltagebetween both electrodes of a capacitance element 1206, that is, thegate-source voltage of the TFT 1204 is turned to be (V_(Sig)−V_(DD)).Then, when the select period of the gate signal line 1202 is finished tobe at high level and the TFTs 1203 and 1205 are turned off, themigration path for charges stored in the capacitance element 1206 isgone and the gate-source voltage (V_(Sig)−V_(DD)) of the TFT 1204 isheld (FIG. 12C).

Here, when (V_(Sig)−V_(DD)) is below the threshold of the TFT 1204, theTFT 1204 is turned on, current is carried through the power source line1209, the EL element 1208 and the current supply line 1207 to startlight emission (FIG. 12D), and the source potential of the TFT 1204drops. At this time, the gate electrode of the TFT 1204 is in thefloating state, and the capacitance element 1206 holds the gate-sourcevoltage of the TFT 1204. Therefore, the potential of the gate electrodealso drops with the decrease in the source potential.

In FIG. 12A, the P-channel TFT is used for all the TFTs configuring thepixel. However, the TFTs 1203 and 1205 are fine to simply function asthe switching elements, as described in the other embodiments. Thus, thepolarity is not defined. In addition, the TFTs 1203 and 1205 do not needto be driven only by the gate signal line 1202. Such the configurationis acceptable that the separate TFTs are controlled by another gatesignal line.

EXAMPLES

Hereafter, the examples of the invention will be described.

Example 1

In this example, the configuration of a light emitting device in whichanalogue video signals are used for video signals for display will bedescribed. FIG. 16A depicts the exemplary configuration of the lightemitting device. The device has a pixel part 1602 where a plurality ofpixels is arranged in a matrix shape over a substrate 1601, and it has asource signal line drive circuit 1603 and first and second gate signalline drive circuits 1604 and 1605 around the pixel part. Two gate signalline drive circuits are so used in FIG. 16A. However, when one gatesignal line is used in the pixel as shown in FIG. 1A, the gate signalline is controlled from both sides simultaneously. When two gate signallines are used in the pixel shown in FIGS. 4A and 5A, the separate gatesignal line drive circuits control the respective gate signal lines.

Signals inputted to the source signal line drive circuit 1603, and thefirst and second gate signal line drive circuits 1604 and 1605 are fedfrom outside through a flexible printed circuit (FPC) 1606.

FIG. 16B depicts the exemplary configuration of the source signal linedrive circuit. This is the source signal line drive circuit for usinganalogue video signals for video signals for display, which has a shiftregister 1611, a buffer 1612, and a sampling circuit 1613. Not shownparticularly, but a level shifter may be added as necessary.

The operation of the source signal line drive circuit will be described.FIG. 17A shows the more detailed configuration, thus referring to thedrawing.

A shift register 1701 is formed of a plurality of flip-flop circuits(FF) 1702, to which the clock signal (S-CLK), the clock inverted signal(S-CLKb), and the start pulse (S-SP) are inputted. In response to thetiming of these signals, sampling pulses are outputted sequentially.

The sampling pulses outputted from the shift register 1701 are passedthrough a buffer 1703 and amplified, and then inputted to a samplingcircuit. The sampling circuit 1704 is formed of a plurality of samplingswitches (SW) 1705, which samples video signals in a certain column inaccordance with the timing of inputting the sampling pulses. Morespecifically, when the sampling pulses are inputted to the samplingswitches, the sampling switches 1705 are turned on. The potential heldby the video signals at this time is outputted to the separate sourcesignal lines through the sampling switches.

Subsequently, the operation of the gate signal line drive circuit willbe described. FIG. 17B depicts the more detailed exemplary configurationof the first and second gate signal line drive circuits 1604 and 1605shown in FIG. 16C. The first gate signal line drive circuit has a shiftregister circuit 1711, and a buffer 1712, which is driven in response tothe clock signal (G-CLK1), the clock inverted signal (G-CLKb1), and thestart pulse (G-SP1). The second gate signal line drive circuit 1605 mayalso be configured similarly.

The operation from the shift register to the buffer is the same as thatin the source signal line drive circuit. The sampling pulses amplifiedby the buffer select separate gate signal lines for them. The first gatesignal line drive circuit sequentially selects first gate signal linesG₁₁, G₂₁, . . . and G_(m1), and the second gate signal line drivecircuit sequentially selects second gate signal lines G₁₂, G₂₂, . . .and G_(m2). A third gate signal line drive circuit, not shown, is alsothe same as the first and second gate signal line drive circuits,sequentially selecting third gate signal lines G₁₃, G₂₃, . . . andG_(m3). In the selected row, video signals are written in the pixel toemit light according to the procedures described in the embodiments.

In addition, as one example of the shift register, that formed of aplurality of D flip-flops is shown here. However, such the configurationis acceptable that signal lines can be selected by a decoder.

Example 2

In this example, the configuration of a light emitting device in whichdigital video signals are used for video signals for display will bedescribed. FIG. 18A depicts the exemplary configuration of a lightemitting device. The device has a pixel part 1802 where a plurality ofpixels is arranged in a matrix shape over a substrate 1801, and it has asource signal line drive circuit 1803, and first and second gate signalline circuits 1804 and 1805 around the pixel part. Two gate signal linedrive circuits are used in FIG. 18A. However, when one gate signal lineis used in the pixel as shown in FIG. 1A, the gate signal line iscontrolled from both sides simultaneously. When two gate signal linesare used in the pixel as shown in FIGS. 4A and 5A, the separate gatesignal line drive circuits control the respective gate signal lines.

Signals inputted to the source signal line drive circuit 1803, and thefirst and second gate signal line drive circuits 1804 and 1805 are fedfrom outside through a flexible printed circuit (FPC) 1806.

FIG. 18B depicts the exemplary configuration of the source signal linedrive circuit. This is the source signal line drive circuit for usingdigital video signals for video signals for display, which has a shiftregister 1811, a first latch circuit 1812, a second latch circuit 1813,and a D/A converter circuit 1814. Not shown in the drawing particularly,but a level shifter may be added as necessary.

The first and second gate signal line drive circuits 1804 and 1805 arefine to be those shown in the example 1, thus omitting the illustrationand description here.

The operation of the source signal line drive circuit will be described.FIG. 19A shows the more detailed configuration, thus referring to thedrawing.

A shift register 1901 is formed of a plurality of flip-flop circuits(FF) 1910, to which the clock signal (S-CLK), the clock inverted signal(S-CLKb), and the start pulse (S-SP) are inputted. Sampling pulses aresequentially outputted in response to the timing of these signals.

The sampling pulses outputted from the shift register 1901 are inputtedto first latch circuits 1902. Digital video signals are being inputtedto the first latch circuits 1902. The digital video signals are held ateach stage in response to the timing of inputting the sampling pulses.Here, the digital video signals are inputted by three bits. The videosignals at each bit are held in the separate first latch circuits. Here,three first latch circuits are operated in parallel by one samplingpulse.

When the first latch circuits 1902 finish to hold the digital videosignals up to the last stage, latch pulses are inputted to second latchcircuits 1903 during the horizontal retrace period, and the digitalvideo signals held in the first latch circuits 1902 are transferred tothe second latch circuits 1903 all at once. After that, the digitalvideo signals held in the second latch circuits 1903 for one row areinputted to D/A converter circuits 1904 simultaneously.

While the digital video signals held in the second latch circuits 1903are being inputted to the D/A converter circuits 1904, the shiftregister 1901 again outputs sampling pulses. Subsequent to this, theoperation is repeated to process the video signals for one frame.

The D/A converter circuits 1904 convert the inputted digital videosignals from digital to analogue and output them to the source signallines as the video signals having the analogue voltage.

The operation described above is conducted throughout the stages duringone horizontal period. Accordingly, the video signals are outputted tothe entire source signal lines.

In addition, as described in the example 1, such the configuration isacceptable that a decoder is used instead of the shift register toselect signal lines.

Example 3

In the example 2, digital video signals are converted from digital toanalogue by the D/A converter circuits and are written in the pixels.The semiconductor device of the invention can also express gray scalesby the time gray scale system. In this case, the D/A converter circuitsare not needed as shown in FIG. 19B, and gray scales are controlled overthe expression by the length of time that the EL element is emittinglight for a long tome or short time. Thus, the video signals of each bitdo not need to undergo parallel processing. Therefore, both the firstand second latch circuits are fine for one bit. At this time, thedigital video signals of each bit are serially inputted, sequentiallyheld in the latch circuits and written in the pixels. Of course, it isacceptable that latch circuits for necessary bits are arranged inparallel.

Example 4

In this specification, a substrate on which a driver circuit, a pixelpart having TFTs for switching and TFTs for driving are formed, for thesake of convenience, is referred to as an active-matrix substrate. Inthis example, the active substrate manufactured by using unipolar TFTswill be described with reference to FIGS. 13 A to 14C.

A quartz substrate, a silicon substrate, a metallic substrate, or astainless substrate, in which an insulating film is formed on thesurface thereof is used as a substrate 5000. In addition, a plasticsubstrate having a heat resistance, which is resistant to a processingtemperature in this manufacturing process may be used. In this example,the substrate 5000 made of glass such as barium borosilicate glass oraluminoborosilicate glass is used.

Next, a base film 5001 made from an insulating film such as a siliconoxide film, a silicon nitride film, or a silicon oxynitride film isformed on the substrate 5000. In this example, a two-layer structure isused for the base film 5001. However, a single layer structure of theinsulating film or a structure in which two layers or more of theinsulating film are laminated may be used.

In this example, as a first layer of the base film 5001, a siliconoxynitride film 5001 a is formed at a thickness of 10 nm to 200 nm(preferably, 50 nm to 100 nm) by a plasma CVD method using SiH₄, NH₃,and N₂O as reactive gases. In this example, the silicon oxynitride film5001 a is formed at a thickness of 50 nm. Next, as a second layer of thebase film 5001, a silicon oxynitride film 5001 b is formed at athickness of 50 nm to 200 nm (preferably, 100 nm to 150 nm) by a plasmaCVD method using SiH₄ and N₂O as reactive gases. In this example, thesilicon oxynitride film 5001 b is formed at a thickness of 100 nm.

Subsequently, semiconductor layers 5002 to 5005 are formed on the basefilm 5001. The semiconductor layers 5002 to 5005 are formed as follows.That is, a semiconductor film is formed at a thickness of 25 nm to 80 nm(preferably, 30 nm to 60 nm) by known means (such as a sputteringmethod, an LPCVD method, or a plasma CVD method). Next, thesemiconductor film is crystallized by a known crystallization method(such as a laser crystallization method, a thermal crystallizationmethod using RTA or a furnace anneal furnace, a thermal crystallizationmethod using a metallic element for promoting crystallization, or thelike). Then, the obtained crystalline semiconductor film is patterned ina predetermined shape to form the semiconductor layers 5002 to 5005.Note that an amorphous semiconductor film, a micro-crystallinesemiconductor film, a crystalline semiconductor film, a compoundsemiconductor film having an amorphous structure such as an amorphoussilicon germanium film, or the like may be used as the semiconductorfilm.

In this example, an amorphous silicon film having a film thickness of 55nm is formed by a plasma CVD method. A solution containing nickel isheld on the amorphous silicon film and it is dehydrogenated at 500° C.for 1 hour, and then thermal crystallization is conducted at 550° C. for4 hours to form a crystalline silicon film. After that, patterningprocessing using a photolithography method is performed to form thesemiconductor layers 5002 to 5005.

Note that, when the crystalline semiconductor film is formed by a lasercrystallization method, a gas laser or a solid laser, which conductscontinuous oscillation or pulse oscillation is preferably used as thelaser. An excimer laser, a YAG laser, a YVO₄ laser, a YLF laser, a YAlO₃laser, a glass laser, a ruby laser, a Ti:sapphire laser, and the likecan be used as the former gas laser. In addition, a laser using acrystal such as YAG, YVO₄, YLF or YAlO₃, which is doped with Cr, Nd, Er,Ho, Ce, Co, Ti, or Tm can be used as the latter solid laser. Thefundamental of the laser is changed according to a doping material andlaser light having a fundamental of the neighborhood of 1 μm isobtained. A harmonic to the fundamental can be obtained by using anon-linear optical element. Note that, in order to obtain a crystalhaving a large grain size at the crystallization of the amorphoussemiconductor film, it is preferable that a solid laser capable ofconducting continuous oscillation is used and a second harmonic to afourth harmonic of the fundamental are applied. Typically, a secondharmonic (532 nm) or a third harmonic (355 nm) of an Nd:YVO₄ laser(fundamental of 1064 nm) is applied.

Also, laser light emitted from the continuous oscillation YVO₄ laserhaving an output of 10 W is converted into a harmonic by a non-linearoptical element. Further, there is a method of locating an YVO₄ crystaland a non-linear optical element in a resonator and emitting a harmonic.Preferably, laser light having a rectangular shape or an ellipticalshape is formed on an irradiation surface by an optical system andirradiated to an object to be processed. At this time, an energy densityof about 0.01 MW/cm² to 100 MW/cm² (preferably, 0.1 MW/cm² to 10 MW/cm²)is required. The semiconductor film is moved relatively to the laserlight at a speed of about 10 cm/s to 2000 cm/s to be irradiated with thelaser light.

Also, when the above laser is used, it is preferable that a laser beamemitted from a laser oscillator is linearly condensed by an opticalsystem and irradiated to the semiconductor film. A crystallizationcondition is set as appropriate. When an excimer laser is used, it ispreferable that a pulse oscillation frequency is set to 300 Hz and alaser energy density is set to 100 mJ/cm² to 700 mJ/cm² (typically, 200mJ/cm² to 300 mJ/cm²). In addition, when a YAG laser is used, it ispreferable that the second harmonic is used, a pulse oscillationfrequency is set to 1 Hz to 300 Hz, and a laser energy density is set to300 mJ/cm² to 1000 mJ/cm² (typically, 350 mJ/cm² to 500 mJ/cm²). A laserbeam linearly condensed at a width of 100 μm to 1000 μm (preferably, 400μm) is irradiated over the entire surface of the substrate. At thistime, an overlap ratio with respect to the linear beam may be set to 50%to 98%.

However, in this example, the amorphous silicon film is crystallizedusing a metallic element for promoting crystallization so that themetallic element remains in the crystalline silicon film. Thus, anamorphous silicon film having a thickness of 50 nm to 100 nm is formedon the crystalline silicon film, heat treatment (thermal anneal using anRTA method or a furnace anneal furnace) is conducted to diffuse themetallic element into the amorphous silicon film, and the amorphoussilicon film is removed by etching after the heat treatment. As aresult, the metallic element contained in the crystalline silicon filmcan be reduced or removed.

Note that, after the formation of the semiconductor layers 5002 to 5005,doping with a trace impurity element (boron or phosphorus) may beconducted in order to control a threshold value of a TFT.

Next, a gate insulating film 5006 covering the semiconductor layers 5002to 5005 is formed. The gate insulating film 5006 is formed from aninsulating film containing silicon at a film thickness of 40 nm to 150nm by a plasma CVD method or a sputtering method. In this example, asilicon oxynitride film is formed as the gate insulating film 5006 at athickness of 115 nm by the plasma CVD method. Of course, the gateinsulating film 5006 is not limited to the silicon oxynitride film.Another insulating film containing silicon may be used as a single layeror a laminate structure.

Note that, when a silicon oxide film is used as the gate insulating film5006, a plasma CVD method is employed, TEOS (tetraethyl orthosilicate)and O₂ are mixed, a reactive pressure is set to 40 Pa, and a substratetemperature is set to 300° C. to 400° C. Then, discharge may occur at ahigh frequency (13.56 MHz) power density of 0.5 W/cm² to 0.8 W/cm² toform the silicon oxide film. After that, when thermal anneal isconducted for the silicon oxide film formed by the above steps at 400°C. to 500° C., a preferable property as to the gate insulating film 5006can be obtained.

Next, a first conductive film 5007 having a film thickness of 20 nm to100 nm and a second conductive film 5008 having a film thickness of 100nm to 400 nm are laminated on the gate insulating film 5006. In thisexample, the first conductive film 5007, which has the film thickness of30 nm and is made from a TaN film and the second conductive film 5008,which has the film thickness of 370 nm and is made from a W film arelaminated.

In this example, the TaN film as the first conductive film 5007 isformed by a sputtering method using Ta as a target in an atmospherecontaining nitrogen. The W film as the second conductive film 5008 isformed by a sputtering method using W as a target. In addition, it canbe formed by a thermal CVD method using tungsten hexafluoride (WF₆). Inany case, when they are used for a gate electrode, it is necessary toreduce a resistance, and it is desirable that a resistivity of the Wfilm is set to 20 μΩ cm or lower. When a crystal grain is enlarged, theresistivity of the W film can be reduced. However, if a large number ofimpurity elements such as oxygen exist in the W film, thecrystallization is suppressed so that the resistance is increased.Therefore, in this example, the W film is formed by a sputtering methodusing high purity W (purity of 99.9999%) as a target while taking into aconsideration that an impurity does not enter the film from a gas phaseat film formation. Thus, a resistivity of 9 μΩ cm to 20 μΩ cm can berealized.

Note that, in this example, the TaN film is used as the first conductivefilm 5007 and the W film is used as the second conductive film 5008.However, materials that compose the first conductive film 5007 and thesecond conductive film 5008 are not particularly limited. The firstconductive film 5007 and the second conductive film 5008 each may beformed from an element selected from Ta, W, Ti, Mo, Al, Cu, Cr, and Nd,or an alloy material or a compound material, which contains mainly theabove element. In addition, they may be formed from a semiconductor filmthat is represented by a polycrystalline silicon film doped with animpurity element such as phosphorus, or an AgPdCu alloy.

Next, a mask 5009 made of a resist is formed by using a photolithographymethod and first etching processing for forming electrodes and wiringsis performed. The first etching processing is performed under a firstetching condition and a second etching condition (FIG. 13B).

In this example, as the first etching condition, an ICP (inductivelycoupled plasma) etching method is used. In addition, CF₄, Cl₂, and O₂are used as etching gases and a ratio of respective gas flow rates isset to 25:25:10 (sccm). RF power having 500 W and 13.56 MHz is suppliedto a coil type electrode at a pressure of 1.0 Pa to produce plasma,thereby conducting etching. RF power having 150 W and 13.56 MHz issupplied to a substrate side (sample stage) to apply a substantiallynegative self bias voltage thereto. The W film is etched under thisfirst etching condition so that end portions of the first conductivelayer 5007 are made to have taper shapes.

Subsequently, the etching condition is changed to the second etchingcondition without removing the mask 5009 made of a resist. CF₄ and Cl₂are used as etching gases and a ratio of respective gas flow rates isset to 30:30 (sccm). RF power having 500 W and 13.56 MHz is supplied toa coil type electrode at a pressure of 1.0 Pa to produce plasma, therebyconducting etching for about 15 seconds. RF power having 20 W and 1356MHz is supplied to a substrate side (sample stage) to apply asubstantially negative self bias voltage thereto. In the second etchingcondition, both the first conductive film 5007 and the second conductivefilm 5008 are etched to the same degree. Note that, in order to conductetching without leaving the residue on the gate insulating film 5006, itis preferable that an etching time is increased at a rate of about 10 to20%.

In the above first etching processing, when a shape of the mask made ofa resist is made suitable, the end portions of the first conductive film5007 and the end portions of the second conductive film 5008 becometaper shapes by an effect of the bias voltage applied to the substrateside. Thus, first-shaped conductive layers 5010 to 5014 made from thefirst conductive layer 5007 and the second conductive layer 5008 areformed by the first etching processing. With respect to the insulatingfilm 5006, regions which are not covered with the first-shapedconductive layers 5010 to 5014 are etched by about 20 nm to 50 nm sothat thinner regions are formed.

Next, second etching processing is performed without removing the mask5009 made of a resist (FIG. 13C). In the second etching processing, SF₆,Cl₂, and O₂ are used as etching gases and a ratio of respective gas flowrates is set to 24:12:24 (sccm). RF power having 700 W and 13.56 MHz issupplied to a coil type electrode at a pressure of 1.3 Pa to produceplasma, thereby conducting etching for about 25 seconds. RF power having10 W and 1356 MHz is supplied to a substrate side (sample stage) toapply a substantially negative self bias voltage thereto. Thus, the Wfilm is selectively etched to form second-shaped conductive layers 5015to 5019. At this time, first conductive layers 5015 a to 5018 a arehardly etched.

Then, first doping processing is performed without removing the mask5009 made of a resist to add an impurity element for providing an N-typeto the semiconductor layers 5002 to 5005 at a low concentration. Thefirst doping processing is preferably performed by an ion doping methodor an ion implantation method. With respect to a condition of the iondoping method, a dose is set to 1×10¹³ atoms/cm² to 5×10¹⁴ atoms/cm² andan accelerating voltage is set to 40 keV to 80 keV. In this example, adose is set to 5.0×10¹³ atoms/cm² and an accelerating voltage is set to50 keV. As the impurity element for providing an N-type, an elementwhich belongs to Group 15 is preferably used, and typically, phosphorus(P) or arsenic (As) is used. In this example, phosphorus (P) is used. Inthis case, the second-shaped conductive layers 5015 to 5019 become masksto the impurity element for providing an N-type. Thus, first impurityregions (N-regions) 5020 to 5023 are formed in a self alignment. Then,the impurity element for providing an N-type is added to the firstimpurity regions 5020 to 5023 at a concentration range of 1×10¹⁸atoms/cm³ to 1×10²⁰ atoms/cm³.

Subsequently, after the mask 5009 made of a resist is removed, a newmask 5024 made of a resist is formed and second doping processing isperformed at a higher accelerating voltage than that in the first dopingprocessing. In a condition of an ion doping method, a dose is set to1×10¹³ atoms/cm² to 3×10¹⁵ atoms/cm² and an accelerating voltage is setto 60 keV to 120 keV. In this example, a dose is set to 3.0×10¹⁵atoms/cm² and an accelerating voltage is set to 65 keV. In the seconddoping processing, second conductive layers 5015 b to 5018 b are used asmasks to an impurity element and doping is conducted such that theimpurity element is added to the semiconductor layers located under thetaper portions of the first conductive layers 5015 a to 5018 a.

As a result of the above second doping processing, the impurity elementfor providing an N-type is added to second impurity regions (N− regions;Lov regions) 5026, 5029 overlapped with the first conductive layers at aconcentration range of 1×10¹⁸ atoms/cm³ to 5×10¹⁹ atoms/cm³. Inaddition, the impurity element for providing an N-type is added to thirdimpurity regions (N+ regions) 5025, 5028, 5031 and 5034 at aconcentration range of 1×10¹⁹ atoms/cm³ to 5×10²¹ atoms/cm³. After thefirst and second doping processings, regions to which no impurityelement is added or regions to which the trace impurity element is addedare formed in the semiconductor layers 5002 to 5005. In this example,the regions to which the impurity element is not completely added or theregions to which the trace impurity element is added are called channelregions 5027, 5030, 5033 and 5036. In addition, there are, of the firstimpurity regions (N−-regions) 5020 to 5023 formed by the above firstdoping processing, regions covered with the resist 5024 in the seconddoping processing. In this example, they are continuously called firstimpurity regions (N−-regions; LDD regions) 5032, 5035.

Note that, in this example, the second impurity regions (N− regions)5026 and the third impurity regions (N+ regions) 5025, 5028, 5031 and5034 are formed by only the second doping processing. However, thepresent invention is not limited to this. A condition for dopingprocessing may be changed as appropriate and doping processing may beperformed plural times to form those regions.

Next, as shown in FIG. 14A, the mask 5024 made of a resist is removedand a first interlayer insulating film 5037 is formed. An insulatingfilm containing silicon is formed as the first interlayer insulatingfilm 5037 at a thickness of 100 nm to 200 nm by a plasma CVD method or asputtering method. In this example, a silicon oxynitride film is formedat a film thickness of 100 nm by a plasma CVD method. Of course, thefirst interlayer insulating film 5037 is not limited to the siliconoxynitride film, and therefore another insulating film containingsilicon may be used as a single layer or a laminate structure.

Next, heat treatment is performed for the recovery of crystallinity ofthe semiconductor layers and the activation of the impurity elementadded to the semiconductor layers. This heat treatment is performed by athermal anneal method using a furnace anneal furnace. The thermal annealmethod is preferably conducted in a nitrogen atmosphere in which anoxygen concentration is 1 ppm or less, preferably, 0.1 ppm or less at400° C. to 700° C. In this example, the heat treatment at 410° C. for 1hour is performed for the activation processing. Note that a laseranneal method or a rapid thermal anneal method (RTA method) can beapplied in addition to the thermal anneal method.

Also, the heat treatment may be performed before the formation of thefirst interlayer insulating film 5037. However, if materials whichcompose the first conductive layers 5015 a to 5019 a and the secondconductive layers 5015 b to 5019 b are sensitive to heat, it ispreferable that heat treatment is performed after the first interlayerinsulating film 5037 (insulating film containing mainly silicon, forexample, silicon nitride film) for protecting a wiring and the like isformed as in this example.

As described above, when the heat treatment is performed after theformation of the first interlayer insulating film 5037 (insulating filmcontaining mainly silicon, for example, silicon nitride film), thehydrogenation of the semiconductor layer can be also conductedsimultaneous with the activation processing. In the hydrogenation step,a dangling bond of the semiconductor layer is terminated by hydrogencontained in the first interlayer insulating film 5037.

Note that heat treatment for hydrogenation which is different from theheat treatment for activation processing may be performed.

Here, the semiconductor layer can be hydrogenated regardless of thepresence or absence of the first interlayer insulating film 5037. Asanother means for hydrogenation, means for using hydrogen excited byplasma (plasma hydrogenation) or means for performing heat treatment inan atmosphere containing hydrogen of 3% to 100% at 300° C. to 450° C.for 1 hour to 12 hours may be used.

Next, a second interlayer insulating film 5038 is formed on the firstinterlayer insulating film 5037. An inorganic insulating film can beused as the second interlayer insulating film 5038. For example, asilicon oxide film formed by a CVD method, a silicon oxide film appliedby an SOG (spin on glass) method, or the like can be used. In addition,an organic insulating film can be used as the second interlayerinsulating film 5038. For example, a film made of polyimide, polyamide,BCB (benzocyclobutene), acrylic, or the like can be used. Further, alaminate structure of an acrylic film and a silicon oxide film may beused.

In this example, an acrylic film having a film thickness of 1.6 lm isformed. When the second interlayer insulating film 5038 is formed,unevenness caused by TFTs formed on the substrate 5000 is reduced andthe surface can be leveled. In particular, the second interlayerinsulating film 5038 has a strong sense of leveling. Thus, a film havingsuperior evenness is preferable.

Next, using dry etching or wet etching, the second interlayer insulatingfilm 5038, the first interlayer insulating film 5037, and the gateinsulating film 5006 are etched to form contact holes which reach theimpurity regions 5025, 5028, 5031 and 5034.

Next, a pixel electrode 5039 made from a transparent conductive film isformed. A compound of indium oxide and tin oxide (indium tin oxide:ITO), a compound of indium oxide and zinc oxide, zinc oxide, tin oxide,indium oxide, or the like can be used for the transparent conductivefilm. In addition, the transparent conductive film to which gallium isadded may be used. The pixel electrode corresponds to the anode of an ELelement.

In this example, an ITO film is formed at a thickness of 110 nm and thenpatterned to form the pixel electrode 5039.

Next, wirings 5040 to 5046 electrically connected with the respectiveimpurity regions are formed. Note that, in this example, a Ti filmhaving a film thickness of 100 nm, an Al film having a film thickness of350 nm, and a Ti film having a film thickness of 100 nm are formed intoa laminate in succession by a sputtering method and a resultant laminatefilm is patterned in a predetermined shape so that the wirings 5040 to5046 are formed.

Of course, they are not limited to a three-layer structure. A singlelayer structure, a two-layer structure, or a laminate structure composedof four layers or more may be used. Materials of the wirings are notlimited to Al and Ti, and therefore other conductive films may be used.For example, an Al film or a Cu film is formed on a TaN film, a Ti filmis formed thereon, and then a resultant laminate film is patterned toform the wirings.

Here, a portion on the pixel electrode 5039 and a portion of the wiring5045 are overlapped with each other so that electrical connectionbetween the wiring 5045 and the pixel electrode 5039 is produced.

By the above steps, as shown in FIG. 14B, the driver circuit portionincluding the N-channel TFT and the pixel portion including theswitching TFT and the driving TFT can be formed on the same substrate.

The N-channel TFT in the driver circuit portion includes lowconcentration impurity regions 5026 (Lov regions) overlapped with thefirst conductive layer 5015 a composing a portion of the gate electrodeand high concentration impurity regions 5025 which each serve as thesource region or the drain region.

The N-channel switching TFT in the pixel portion includes lowconcentration impurity regions 5032 (Loff regions) formed outside thegate electrode and high concentration impurity regions 5031 which eachserve as the source region or the drain region.

Next, a third interlayer insulating film 5047 is formed. An inorganicinsulating film or an organic insulating film can be used as the thirdinterlayer insulating film 5047. A silicon oxide film formed by a CVDmethod, a silicon oxide film applied by an SOG (spin on glass) method,or a silicon oxynitride formed by a sputtering method or the like can beused as the inorganic insulating film. In addition, an acrylic resinfilm or the like can be used as the organic insulating film.

Examples of a combination of the second interlayer insulating film 5038and the third interlayer insulating film 5047 will be described below.

There is a combination in which a silicon oxynitride film formed by anacrylic and a sputtering method is used as the second interlayerinsulating film 5038 and a silicon oxynitride film formed by asputtering method is used as the third interlayer insulating film 5047.In addition, there is a combination in which a silicon oxide film formedby an SOG method is used as the second interlayer insulating film 5038and a silicon oxide film formed by an SOG method is used as the thirdinterlayer insulating film 5047. In addition, there is a combination inwhich a laminate film of a silicon oxide film formed by an SOG methodand a silicon oxide film formed by a plasma CVD method is used as thesecond interlayer insulating film 5038 and a silicon oxide film formedby a plasma CVD method is used as the third interlayer insulating film5047. In addition, there is a combination in which acrylic is used forthe second interlayer insulating film 5038 and acrylic is used for thethird interlayer insulating film 5047. In addition, there is acombination in which a laminate film of an acrylic film and a siliconoxide film formed by a plasma CVD method is used as the secondinterlayer insulating film 5038 and a silicon oxide film formed by aplasma CVD method is used as the third interlayer insulating film 5047.In addition, there is a combination in which a silicon oxide film formedby a plasma CVD method is used as the second interlayer insulating film5038 and acrylic is used for the third interlayer insulating film 5047.

An opening portion is formed at a position corresponding to the pixelelectrode 5039 in the third interlayer insulating film 5047. The thirdinterlayer insulating film serves as a bank. When a wet etching methodis used at the formation of the opening portion, it can be easily formedas a side wall having a taper shape. If the side wall of the openingportion is not sufficiently gentle, the deterioration of an EL layer bya step becomes a marked problem. Thus, attention is required.

A carbon particle or a metallic particle may be added into the thirdinterlayer insulating film 5047 to reduce resistivity, therebysuppressing the generation of static electricity. At this time, theamount of carbon particle or metallic particle to be added is preferablyadjusted such that the resistivity becomes 1×10⁶ Ωm to 1×10¹² Ωm(preferably, 1×10⁸ Ωm to 1×10¹⁰ Ωm).

Next, an EL layer 5048 is formed on the pixel electrode 5039 exposed inthe opening portion of the third interlayer insulating film 5047.

An organic light emitting material or an inorganic light emittingmaterial which are known can be used as the EL layer 5048.

A low molecular weight based organic light emitting material, a highmolecular weight based organic light emitting material, or a mediummolecular weight based organic light emitting material can be freelyused as the organic light emitting material. Note that in thisspecification, a medium molecular weight based organic light emittingmaterial indicates an organic light emitting material which has nosublimation property and in which the number of molecules is 20 or lessor a length of chained molecules is 101 m or less.

The EL layer 5048 has generally a laminate structure. Typically, thereis a laminate structure of “a hole transporting layer, a light emittinglayer, and an electron transporting layer”, which has been proposed byTang et al. in Eastman Kodak Company. In addition to this, a structurein which “a hole injection layer, a hole transporting layer, a lightemitting layer, and an electron transporting layer” or “a hole injectionlayer, a hole transporting layer, a light emitting layer, an electrontransporting layer, and an electron injection layer” are laminated on ananode in this order may be used. A light emitting layer may be dopedwith fluorescent pigment or the like.

In this example, the EL layer 5048 is formed by an evaporation methodusing a low molecular weight based organic light emitting material.Specifically, a laminate structure in which a copper phthalocyanine(CuPc) film having a thickness of 20 nm is provided as the holeinjection layer and a tris-8-quinolinolato aluminum complex (Alq₃) filmhaving a thickness of 70 nm is provided thereon as the light emittinglayer is used. A light emission color can be controlled by addingfluorescent pigment such as quinacridon, perylene, or DCM1 to Alq₃.

Note that only one pixel is shown in FIG. 14C. However, a structure inwhich the EL layers 5048 corresponding to respective colors of, pluralcolors, for example, R (red), G (green), and B (blue) are separatelyformed can be used.

Also, as an example using the high molecular weight based organic lightemitting material, the EL layer 5048 may be constructed by a laminatestructure in which a polythiophene (PEDOT) film having a thickness of 20nm is provided as the hole injection layer by a spin coating method anda paraphenylenevinylene (PPV) film having a thickness of about 100 nm isprovided thereon as the light emitting layer. When π conjugated systempolymer of PPV is used, a light emission wavelength from red to blue canbe selected. In addition, an inorganic material such as silicon carbidecan be used as the electron transporting layer and the electroninjection layer.

Note that the EL layer 5048 is not limited to a layer having a laminatestructure in which the hole injection layer, the hole transportinglayer, the light emitting layer, the electron transporting layer, theelectron injection layer, and the like are distinct. In other words, theEL layer 5048 may have a laminate structure with a layer in whichmaterials composing the hole injection layer, the hole transportinglayer, the light emitting layer, the electron transporting layer, theelectron injection layer, and the like are mixed.

For example, the EL layer 5048 may have a structure in which a mixedlayer composed of a material composing the electron transporting layer(hereinafter referred to as an electron transporting material) and amaterial composing the light emitting layer (hereinafter referred to asa light emitting material) is located between the electron transportinglayer and the light emitting layer.

Next, a pixel electrode 5049 made from a conductive film is provided onthe EL layer 5048. In the case of this example, an alloy film ofaluminum and lithium is used as the conductive film. Of course, a knownMgAg film (alloy film of magnesium and silver) may be used. The pixelelectrode 5049 corresponds to the cathode of the EL element. Aconductive film made of an element which belongs to Group 1 or Group 2of the periodic table or a conductive film to which those elements areadded can be freely used as a cathode material.

When the pixel electrode 5049 is formed, the EL element is completed.Note that the EL element indicates an element composed of the pixelelectrode (anode) 5039, the EL layer 5048, and the pixel electrode(cathode) 5049.

It is effective that a passivation film 5050 is provided to completelycover the EL element. A single layer of an insulating film such as acarbon film, a silicon nitride film, or a silicon oxynitride film, or alaminate layer of a combination thereof can be used as the passivationfilm 5050.

It is preferable that a film having good coverage is used as thepassivation film 5050, and it is effective to use a carbon film,particularly, a DLC (diamond like carbon) film and a CN film. The DLCfilm can be formed at a temperature range of from a room temperature to100° C. Thus, a film can be easily formed over the EL layer 5047 havinga low heat-resistance. In addition, the DLC film has a high blockingeffect to oxygen so that the oxidization of the EL layer 5048 can besuppressed.

Note that, it is effective that steps up to the formation of thepassivation film 5050 after the formation of the third interlayerinsulating film 5047 are conducted in succession using a multi-chambertype (or in-line type) film formation apparatus without being exposed toair.

Note that, actually, when it is completed up to the state shown in FIG.14C, in order not to be exposed to air, it is preferable that packaging(sealing) is conducted using a protective film (laminate film,ultraviolet curable resin film, or the like) or a transparent scalingmember which has a high airtight property and low degassing. At thistime, when an inner portion surrounded by the sealing member is made toan inert atmosphere or a hygroscopic material (for example, bariumoxide) is located in the inner portion, the reliability of the ELelement is improved.

Also, after an airtightness level is increased by processing such aspackaging, a connector (flexible printed circuit: FPC) for connectingterminals led from elements or circuits which are formed on thesubstrate 5000 with external signal terminals is attached so that it iscompleted as a product.

Also, according to the steps described in this example, the number ofphoto masks required for manufacturing a semiconductor device can bereduced. As a result, the process is shortened and it can contribute tothe reduction in manufacturing cost and the improvement of a yield.

Example 5

In this example, an example in which a semiconductor device ismanufactured according to the present invention will be described usingFIGS. 15A to 15C.

FIG. 15A is a top view of a semiconductor device produced by sealing anelement substrate in which TFTs are formed with a sealing member. FIG.15B is a cross sectional view along a line A-A′ in FIG. 15A. FIG. 15C isa cross sectional view along a line B-B′ in FIG. 15A.

A seal member 4009 is provided to surround a pixel portion 4002, asource signal line driver circuit 4003, and first and second gate signalline driver circuits 4004 a and 4004 b which are provided on a substrate4001. In addition, a sealing member 4008 is provided over the pixelportion 4002, the source signal line driver circuit 4003, and the firstand second gate signal line driver circuits 4004 a and 4004 b. Thus, thepixel portion 4002, the source signal line driver circuit 4003, and thefirst and second gate signal line driver circuits 4004 a and 4004 b aresealed with the substrate 4001, the seal member 4009 and the sealingmember 4008 and filled with a filling agent 4210.

Also, the pixel portion 4002, the source signal line driver circuit4003, and the first and second gate signal line driver circuits 4004 aand 4004 b which are provided on the substrate 4001 each have aplurality of TFTs. In FIG. 15B, TFTs (note that an N-channel TFT and aP-channel TFT are shown here) 4201 included in the source signal linedriver circuit 4003 and a TFT 4202 included in the pixel portion 4002,which are formed on a base film 4010 are typically shown.

An interlayer insulating film (planarization film) 4301 is formed on theTFTs 4201 and 4202, and a pixel electrode (anode) 4203 electricallyconnected with the drain of the TFT 4202 is formed thereon. Atransparent conductive film having a large work function is used as thepixel electrode 4203. A compound of indium oxide and tin oxide, acompound of indium oxide and zinc oxide, zinc oxide, tin oxide, orindium oxide can be used for the transparent conductive film. Inaddition, the transparent conductive film to which gallium is added maybe used.

An insulating film 4302 is formed on the pixel electrode 4203. Anopening portion is formed in the insulating film 4302 on the pixelelectrode 4203. In the opening portion, an organic light emitting layer4204 is formed on the pixel electrode 4203. An organic light emittingmaterial or an inorganic light emitting material which are known can beused as the organic light emitting layer 4204. In addition, the organiclight emitting material includes a low molecular weight based (monomersystem) material and a high molecular weight based (polymer system)material, and any material may be used.

An evaporation technique or an applying method technique which are knownis preferably used as a method of forming the organic light emittinglayer 4204. In addition, a laminate structure or a single layerstructure which is obtained by freely combining a hole injection layer,a hole transporting layer, a light emitting layer, an electrontransporting layer, and an electron injection layer is preferably usedas the structure of the organic light emitting layer.

A cathode 4205 made from a conductive film having a light shieldingproperty (typically, a conductive film containing mainly aluminum,copper, or silver, or a laminate film of the conductive film and anotherconductive film) is formed on the organic light emitting layer 4204. Inaddition, it is desirable that moisture and oxygen which exist in aninterface between the cathode 4205 and the organic light emitting layer4204 are minimized. Thus, a devise is required in which the organiclight emitting layer 4204 is formed in a nitrogen atmosphere or a nobleatmosphere and the cathode 4205 without being exposed to oxygen andmoisture is formed. In this example, the above film formation ispossible by using a multi-chamber type (cluster tool type) filmformation apparatus. A predetermined voltage is supplied to the cathode4205.

By the above steps, a light emitting element 4303 composed of the pixelelectrode (anode) 4203, the organic light emitting layer 4204, and thecathode 4205 is formed. A protective film 4209 is formed on theinsulating film 4302 so as to cover the light emitting element 4303. Theprotective film 4209 is effective to prevent oxygen, moisture, and thelike from penetrating the light emitting element 4303.

Reference numeral 4005 a denotes a lead wiring connected with a powersource, which is connected with a first electrode of the TFT 4202. Thelead wiring 4005 a is passed between the seal member 4009 and thesubstrate 4001 and electrically connected with an FPC wiring 4301 of anFPC 4006 through an anisotropic conductive film 4300.

A glass material, a metallic member (typically, a stainless member), aceramic member, a plastic member (including a plastic film) can be usedas the sealing member 4008. An FRP (fiberglass reinforced plastic)plate, a PVF (polyvinyl fluoride) film, a Mylar film, a polyester film,or an acrylic resin film can be used as the plastic member. In addition,a sheet having a structure in which aluminum foil is sandwiched by a PVFfilm and a Mylar film can be used.

Note that, when a radiation direction of light from the light emittingelement is toward a cover member side, it is required that the covermember is transparent. In this case, a transparent material such as aglass plate, a plastic plate, a polyester film, or acrylic film is used.

Also, in addition to an inert gas such as nitrogen or argon, ultravioletcurable resin or thermal curable resin can be used for the filling agent4210. PVC (polyvinyl chloride), acrylic, polyimide, epoxy resin, siliconresin, PVB (polyvinyl butyral), or EVA (ethylene vinyl acetate) can beused. In this example, nitrogen is used for the filling agent.

Also, in order to expose the filling agent 4210 to a hygroscopicmaterial (preferably barium oxide) or a material capable of absorbingoxygen, a concave portion 4007 is provided to the surface of the sealingmember 4008 in the substrate 4001 side, and the hygroscopic material orthe material capable of absorbing oxygen which is indicated by 4207 islocated. In order to prevent the material 4207 having a hygroscopicproperty or being capable of absorbing oxygen from flying off, thematerial 4207 having a hygroscopic property or being capable ofabsorbing oxygen is held in the concave portion 4007 by a concave covermember 4208. Note that concave cover member 4208 is formed in a finemeshed shape and constructed such that it transmits air and moisture butdoes not transmit the material 4207 having a hygroscopic property orbeing capable of absorbing oxygen. When the material 4207 having ahygroscopic property or being capable of absorbing oxygen is provided,the deterioration of the light emitting element 4303 can be suppressed.

As shown in FIG. 15C, a conductive film 4203 a is formed on the leadwiring 4005 a such that it is in contact with the lead wiring 4005 asimultaneously with the formation of the pixel electrode 4203.

Also, the anisotropic conductive film 4300 has a conductive filler 4300a. When the substrate 4001 and the FPC 4006 are bonded to each other bythermal compression, the conductive film 4203 a located over thesubstrate 4001 and the FPC wiring 4301 located on the FPC 4006 areelectrically connected with each other through the conductive filler4300 a.

Example 6

In this example, an external light emitting quantum efficiency can beremarkably improved by using an organic light emitting material by whichphosphorescence from a triplet excitation can be employed for emitting alight. As a result, the power consumption of light emitting element canbe reduced, the lifetime of light emitting element can be elongated andthe weight of light emitting element can be lightened.

The following is a report where the external light emitting quantumefficiency is improved by using the triplet excitation (T. Tsutsui, C.Adachi, S. Saito, Photochemical processes in Organized MolecularSystems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an organic light emitting material (coumarinpigment) reported by the above article is represented as follows.

-   (M. A. Baldo, D. F. O′ Brien, Y. You, A. Shoustikov, S.    Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1998) p. 151)

The molecular formula of an organic light emitting material (Pt complex)reported by the above article is represented as follows.

-   (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.    Forrest, Appl. Phys. Lett., 75 (1999) p. 4.)-   (T. Tutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T.    Thuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38    (12B) (1999) L1502)

The molecular formula of an organic light emitting material (Ir complex)reported by the above article is represented as follows.

As described above, if phosphorescence from a triplet excitation can beput to practical use, it can realize the external light emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet excitation in principle.

Example 7

The light emitting device using the light emitting element is of theself-emission type, and thus exhibits more excellent recognizability ofthe displayed image in a light place as compared to the liquid crystaldisplay device. Furthermore, the light emitting device has a widerviewing angle. Accordingly, the light emitting device can be applied toa display portion in various electronic apparatuses.

Such electronic apparatuses using a light emitting device of the presentinvention include a video camera, a digital camera, a goggles-typedisplay (head mount display), a navigation system, a sound reproductiondevice (a car audio equipment and an audio set), a lap-top computer, agame machine, a portable information terminal (a mobile computer, amobile phone, a portable game machine, an electronic book, or the like),an image reproduction device including a recording medium (morespecifically, an device which can reproduce a recording medium such as adigital versatile disc (DVD) and so forth, and includes a display fordisplaying the reproduced image), or the like. In particular, in thecase of the portable information terminal, use of the light emittingdevice is preferable, since the portable information terminal that islikely to be viewed from a tilted direction is often required to have awide viewing angle. FIG. 20 respectively shows various specific examplesof such electronic apparatuses.

FIG. 20A illustrates a light emitting display device which includes acasing 3001, a support table 3002, a display portion 3003, a speakerportion 3004, a video input terminal 3005 or the like. The presentinvention is applicable to the display portion 3003. The light emittingdevice is of the self-emission-type and therefore requires no backlight.Thus, the display portion thereof can have a thickness thinner than thatof the liquid crystal display device. The light emitting display deviceis including the entire display device for displaying information, suchas a personal computer, a receiver of TV broadcasting and an advertisingdisplay.

FIG. 20B illustrated a digital still camera which includes a main body3101, a display portion 3102, an image receiving portion 3103, anoperation key 3104, an external connection port 3105, a shutter 3106, orthe like. The light emitting device in accordance with the presentinvention can be used as the display portion 3102.

FIG. 20C illustrates a lap-top computer which includes a main body 3201,a casing 3202, a display portion 3203, a keyboard 3204, an externalconnection port 3205, a pointing mouse 3206, or the like. The lightemitting device in accordance with the present invention can be used asthe display portion 3203.

FIG. 20D illustrated a mobile computer which includes a main body 3301,a display portion 3302, a switch 3303, an operation key 3304, aninfrared port 3305, or the like. The light emitting device in accordancewith the present invention can be used as the display portion 3302.

FIG. 20E illustrates a portable image reproduction device including arecording medium (more specifically, a DVD reproduction device), whichincludes a main body 3401, a casing 3402, a display portion A 3403,another display portion B 3404, a recording medium (DVD or the like)reading portion 3405, an operation key 3406, a speaker portion 3407 orthe like. The display portion A 3403 is used mainly for displaying imageinformation, while the display portion B 3404 is used mainly fordisplaying character information. The light emitting device inaccordance with the present invention can be used as these displayportions A 3403 and B 3404. The image reproduction device including arecording medium further includes a game machine or the like.

FIG. 20F illustrates a goggle type display (head mounted display) whichincludes a main body 3501, a display portion 3502, arm portion 3503 orthe like. The light emitting device in accordance with the presentinvention can be used as the display portion 3502.

FIG. 20G illustrates a video camera which includes a main body 3601, adisplay portion 3602, a casing 3603, an external connecting port 3604, aremote control receiving portion 3605, an image receiving portion 3606,a battery 3607, a sound input portion 3608, an operation key 3609, aneyepiece 3610, or the like. The light emitting device in accordance withthe present invention can be used as the display portion 3602.

FIG. 20H illustrates a mobile phone which includes a main body 3701, acasing 3702, a display portion 3703, a sound input portion 3704, a soundoutput portion 3705, an operation key 3706, an external connecting port3707, an antenna 3708, or the like. The light emitting device inaccordance with the present invention can be used as the display portion3703. Note that the display portion 3703 can reduce power consumption ofthe mobile telephone by displaying white-colored characters on ablack-colored background.

When a brighter luminance of light emitted from the organic lightemitting material becomes available in the future, the light emittingdevice in accordance with the present invention will be applicable to afront-type or rear-type projector in which light including output imageinformation is enlarged by means of lenses or the like to be projected.

The aforementioned electronic apparatuses are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system), and in particular likely todisplay moving picture information. The light emitting device issuitable for displaying moving pictures since the organic light emittingmaterial can exhibit high response speed.

A portion of the light emitting device that is emitting light consumespower, so it is desirable to display information in such a manner thatthe light emitting portion therein becomes as small as possible.Accordingly, when the light emitting device is applied to a displayportion which mainly displays character information, e.g., a displayportion of a portable information terminal, and more particular, aportable telephone or a sound reproduction device, it is desirable todrive the light emitting device so that the character information isformed by a light emitting portion while a non-emission portioncorresponds to the background.

As set forth above, the present invention can be applied variously to awide range of electronic apparatuses in all fields. The electronicapparatuses in this example can be obtained by utilizing a lightemitting device having the structure in which the structures in Example1 through 6 are freely combined.

Example 8

In this example, the top view of the pixel configuration shown in FIG.21A will be described with FIG. 22.

In FIG. 22, a plurality of active layers is formed by patterning thesame layer in the region to form TFTs. Then, the first gate line 2102,the second gate line 2103, and the gate electrodes for the separate TFTsare formed by patterning the same layer. Subsequently, the source signallines 2101 and the current supply line 2108 are formed by patterning thesame layer. Lastly, the first electrode (it is the anode here) of the ELelement (light emitting element) is formed.

Then, a selecting TFT 2104 that a part of the first gate line 2102 isthe gate electrode is disposed. The TFT 2104 is formed to have thedouble gate structure where two gate electrodes are formed in one activelayer, whereby it allows surer selecting (switching) than the singlegate structure where one gate electrode is formed in one active layer.In addition, the TFT 2104 can also be formed to have the multi-gatestructure where three or more gate electrodes are formed in one activelayer.

Furthermore, the channel length (L) of the TFT 2105 is set longer inorder to reduce variations in the TFTs. Moreover, L is further longer,whereby the saturation region of the TFT is allowed to be flat.

Besides, the TFT 2106 having the gate electrode connected to the secondgate line 2103 through contacts is formed. Additionally, the capacitanceelement 2107 formed of the active layer and the same layer as a scanningline is disposed.

For such the configuration of each of the TFTs, it is acceptable thatthe top gate structure where the gate electrode is laid over asemiconductor film (channel forming region) or the bottom gate structurereverse to this is used, and the offset structure or the GOLD structureis used for impurity regions (the source region or drain region).

According to the invention, in the semiconductor device configured ofthe unipolar TFTs, the N-channel TFTs particularly excellent in theelectric characteristics as devices, such the configuration is formedthat variations in the gate-source voltage of the driving TFT are notgenerated due to the deteriorated EL element, whereby hardly allowingthe luminance to be reduced even when the EL element is deteriorated. Inaddition, the configuration proposed in the invention does not needeither to be a complex configuration or to increase the number ofelements forming the pixel. Therefore, it can be applied without causingdemerits such as a decrease in the numerical aperture, thus beinggreatly useful.

Although the present invention has been described in conjunction withthe preferred embodiments, the present invention should not be limitedto these embodiments. For example, transistors used in the presentinvention may be any one of a thin film transistor (TFT) in which anactive layer is made of crystalline semiconductor or amorphoussemiconductor, a single crystal transistor, or a transistor using anorganic semiconductor material as an active layer thereof. For example,a transistor formed by using the SOI technique may be used as a singlecrystal thin film transistor, and a thin film transistor comprisingpoly-silicon or amorphous silicon may be used as the thin filmtransistor.

What is claimed is:
 1. A display device comprising: a pixel comprising afirst transistor, a second transistor, a third transistor, a fourthtransistor, and a light-emitting element, wherein one of a source and adrain of the first transistor is electrically connected to a gateelectrode of the second transistor, wherein one of a source and a drainof the second transistor is directly connected to the light-emittingelement, wherein the other of the source and the drain of the secondtransistor is directly connected to one of a source and a drain of thefourth transistor, wherein one of a source and a drain of the thirdtransistor is directly connected to the light-emitting element, whereinthe other of the source and the drain of the third transistor isdirectly connected to a wiring, wherein the other of the source and thedrain of the fourth transistor is electrically connected to a currentsupply line which is different from the wiring, wherein the other of thesource and the drain of the first transistor is electrically connectedto a source signal line which is different from the wiring and thecurrent supply line, wherein a gate electrode of the first transistor iselectrically connected to a first gate signal line, and wherein a gateelectrode of the fourth transistor is electrically connected to a secondgate signal line.
 2. The display device according to claim 1, whereinthe pixel further comprises a capacitor, wherein one electrode of thecapacitor is electrically connected to the gate electrode of the secondtransistor, and wherein the other electrode of the capacitor iselectrically connected to the light-emitting element.
 3. The displaydevice according to claim 1, wherein the wiring is a power line.
 4. Thedisplay device according to claim 1, wherein each of the firsttransistor, the second transistor, the third transistor, and the fourthtransistor is an N-channel transistor.
 5. The display device accordingto claim 1, wherein each of the first transistor, the second transistorand the fourth transistor comprises a crystalline semiconductor film. 6.The display device according to claim 1, wherein the display device is alight-emitting display device.
 7. The display device according to claim1, wherein a gate electrode of the third transistor is electricallyconnected to the first gate signal line.
 8. The display device accordingto claim 1, wherein each of the gate electrode of the first transistor,the gate electrode of the second transistor, a gate electrode of thethird transistor, and the gate electrode of the fourth transistorcomprises a conductive film, and wherein the conductive film comprisesan element selected from Ta, W, Ti, Mo, Al, Cu, Cr, and Nd.
 9. A displaydevice comprising: a pixel comprising a first transistor, a secondtransistor, a third transistor, and a light-emitting element, whereinone of a source and a drain of the second transistor is directlyconnected to the light-emitting element, wherein one of a source and adrain of the third transistor is directly connected to thelight-emitting element, wherein the other of the source and the drain ofthe third transistor is directly connected to a wiring, wherein theother of the source and the drain of the second transistor is directlyconnected to a current supply line which is different from the wiring,wherein one of a source and a drain of the first transistor iselectrically connected to a gate electrode of the second transistor,wherein the other of the source and the drain of the first transistor iselectrically connected to a source signal line which is different fromthe wiring and the current supply line, wherein a gate electrode of thefirst transistor is electrically connected to a first gate signal line,wherein a gate electrode of the third transistor is electricallyconnected to a second gate signal line, and wherein each of the firsttransistor, the second transistor, and the third transistor is anN-channel transistor.
 10. The display device according to claim 9,wherein the second transistor is a driving transistor.
 11. The displaydevice according to claim 9, wherein a potential between the gateelectrode of the second transistor and the source of the secondtransistor is held by a capacitor.
 12. The display device according toclaim 9, wherein the pixel further comprises a capacitor, wherein oneelectrode of the capacitor is electrically connected to the gateelectrode of the second transistor, and wherein the other electrode ofthe capacitor is electrically connected to the light-emitting element.13. The display device according to claim 9, wherein the wiring is apower line.
 14. The display device according to claim 9, wherein each ofthe gate electrode of the first transistor, the gate electrode of thesecond transistor, and the gate electrode of the third transistorcomprises a first conductive film and a second conductive film, andwherein the first conductive film and the second conductive filmcomprises an element selected from Ta, W, Ti, Mo, Al, Cu, Cr, and Nd.15. The display device according to claim 9, wherein the display deviceis a light-emitting display device.
 16. A display device comprising: apixel comprising a first transistor, a second transistor, a thirdtransistor, and a light-emitting element, wherein one of a source and adrain of the second transistor is directly connected to thelight-emitting element, wherein one of a source and a drain of the thirdtransistor is directly connected to the light-emitting element, whereinthe other of the source and the drain of the third transistor iselectrically connected to a wiring, wherein the other of the source andthe drain of the second transistor is directly connected to a currentsupply line which is different from the wiring, wherein one of a sourceand a drain of the first transistor is electrically connected to a gateelectrode of the second transistor, wherein the other of the source andthe drain of the first transistor is electrically connected to a sourcesignal line which is different from the wiring and the current supplyline, wherein a gate electrode of the first transistor is electricallyconnected to a first gate signal line, wherein a gate electrode of thethird transistor is electrically connected to a second gate signal line,wherein a first semiconductor film comprises a channel formation regionof the first transistor, wherein a second semiconductor film comprises achannel formation region of the second transistor and a channelformation region of the third transistor, and wherein the firstsemiconductor film is separated from the second semiconductor film. 17.The display device according to claim 16, wherein each of the firsttransistor, the second transistor, and the third transistor is anN-channel transistor.
 18. A display device comprising: a pixelcomprising a first transistor, a second transistor, a third transistor,a capacitor, and a light-emitting element, wherein one of a source and adrain of the second transistor is directly connected to thelight-emitting element, wherein one of a source and a drain of the thirdtransistor is directly connected to the light-emitting element, whereinthe other of the source and the drain of the third transistor iselectrically connected to a wiring, wherein the other of the source andthe drain of the second transistor is directly connected to a currentsupply line which is different from the wiring, wherein one of a sourceand a drain of the first transistor is electrically connected to a gateelectrode of the second transistor, wherein the other of the source andthe drain of the first transistor is electrically connected to a sourcesignal line which is different from the wiring and the current supplyline, wherein a gate electrode of the first transistor is electricallyconnected to a first gate signal line, wherein a gate electrode of thethird transistor is electrically connected to a second gate signal line,wherein one electrode of the capacitor is electrically connected to thegate electrode of the second transistor, wherein a first semiconductorfilm comprises a channel formation region of the first transistor,wherein a second semiconductor film comprises a channel formation regionof the second transistor, a channel formation region of the thirdtransistor, and a region functioning as the other electrode of thecapacitor, and wherein the first semiconductor film is separated fromthe second semiconductor film.
 19. The display device according to claim18, wherein each of the first transistor, the second transistor, and thethird transistor is an N-channel transistor.